Painting 3D printed plastic parts: a technical guide

Introduction. 3D printing with plastic is becoming more and more popular, and many people want to know how to paint the finished parts properly. Painting not only improves the look of an object, it also makes it last longer by protecting the plastic from ultraviolet light and moisture. The approach to painting, however, depends on the type of plastic and on how the surface has been prepared. In this guide we go through the main 3D printing materials, the ways of preparing them (from sanding to chemical smoothing) and the primers, paints and varnishes that suit them.

Key terms

FDM (Fused Deposition Modeling) – a 3D printing technology in which a plastic filament is melted and laid down layer by layer to build an object.

SLA (Stereolithography) – a 3D printing technology that uses a liquid photopolymer resin cured (polymerized) by a UV laser.

SLS (Selective Laser Sintering) – a 3D printing technology in which a laser sinters particles of thermoplastic powder layer by layer.

DLP (Digital Light Processing) – a variant of stereolithography in which a UV projector cures a whole layer of resin at once.

Glass transition temperature (Tg) – the temperature at which a polymer passes from a hard, glassy state into a soft, rubbery one. A key figure for judging the heat resistance of a material.

Shrinkage – the reduction in size of a printed part as it cools, caused by thermal contraction. It can lead to distortion and warping.

Anisotropy – mechanical properties (strength) that differ depending on direction. Typical of FDM printing, where the bond between layers is weaker than the bond inside a single layer.

Thermoplastics – plastics that can be melted and solidified over and over again by heating and cooling (PLA, ABS, PETG and so on).

Thermosets (thermosetting plastics) – plastics that form an irreversible cross-linked structure when they cure (often under heat or light) and cannot be melted again (epoxy and photopolymer resins, for example).

Adhesion – the ability of a paint, primer or adhesive to bond firmly to the surface of a material. Poor adhesion is a common problem when painting certain plastics.

The main 3D printing technologies

There are three main technologies for 3D printing with plastic:

FDM (Fused Deposition Modeling)

How it works: the printer melts a thermoplastic filament in the extruder and lays the molten material down on the build plate layer by layer to form the object. Each new layer goes onto the previous one, on the same principle as a hot glue gun.

Main features: the most affordable and most widespread technology for home use. Parts have visible layer lines. Complex shapes with overhangs need supports.

Drawbacks: low print resolution, which limits complex structures. Voids sometimes remain between the layers, which makes parts anisotropic — their strength depends on the direction of the load.

Typical materials: PLA, ABS, PETG, TPU, HIPS, nylon (PA) and various composites.

SLA (Stereolithography) / DLP / LCD

SLA (stereolithography), DLP and LCD are all photopolymer 3D printing methods that work on the same principle. The difference is in the light source.

How it works: in SLA a laser beam scans point by point and cures (polymerizes) liquid photopolymer resin in a vat, forming the cross-section of the part. LCD uses UV LEDs and an LCD mask (a screen) that forms pixels, while DLP uses a projector that projects the whole layer at once.

Main features: the highest level of detail and the smoothest surface of any consumer printer. Well suited to hobby printing. Parts printed with SLA are isotropic — their strength stays the same regardless of orientation, because chemical bonds form between the layers. This gives parts with predictable mechanical properties.

Drawbacks: possible dimensional inaccuracy, processing defects, reduced mechanical properties and incomplete curing, which may be caused by scattering of the ultraviolet light. Post-processing is required: washing off the resin residue and a final UV cure.

Typical materials: photopolymer resins (standard, flexible, engineering, castable, biocompatible).

SLS (Selective Laser Sintering)

How it works: a powerful laser selectively sinters (fuses) particles of thermoplastic powder to form a layer of the part. A new layer of powder is spread and the process repeats. Unsintered powder acts as natural support.

Main features: low cost per part and high throughput. It can produce strong parts with complex geometry without supports. Parts are isotropic and have practically no visible layer lines.

Drawbacks: SLS parts have a grainy surface and internal porosity, which may call for further treatment if a smooth surface or water tightness is needed. SLS cannot print large flat surfaces and small holes accurately, because they are prone to distortion and warping. It is mainly an industrial technology aimed at short production runs of functional parts and is less accessible to hobbyists.

Typical materials: polyamide powder (PA11, PA12), TPU (TPE) powder and metals.

The FDM, SLA and SLS printing technologies

The main plastics for 3D printing

Choosing the right material is the key to success in 3D printing. The type of plastic determines not only the print settings but also the final properties of the object: its strength, heat resistance, appearance and — no less important — how well it can be post-processed and painted. This section covers the main materials used in today's additive technologies.

All the polymers used in 3D printing fall into two fundamental groups that differ in their very nature.

Thermoplastics are the most common class of material for fused deposition modeling (FDM). Their distinguishing feature is that they can pass from the solid state into a melt and back many times on heating and cooling without irreversible chemical breakdown. This is what makes them recyclable and reusable. The popular filaments belong here: PLA, ABS, PETG, nylon and others. An analogy: butter, which can be melted, set solid and melted again.

Thermosets (thermosetting plastics) form an irreversibly cross-linked polymer network once they cure, whether the cure is started by heat, light or other radiation. Such a material does not melt when it is reheated — it decomposes. This class includes the photopolymer resins used in SLA and DLP. Their key advantage is the highest level of detail in the print. An analogy: cake batter, which cannot be turned back into batter once it is baked.

The main FDM thermoplastic filaments

PLA (polylactic acid) — the material for beginners

A biodegradable thermoplastic polyester made from renewable raw materials (corn starch, sugar cane). Its low printing temperature, absence of a sharp smell and minimal shrinkage make it the natural choice for newcomers.

Temperatures: extruder 180–220°C. Build plate 0–60°C (heating is not essential, but it is recommended for better adhesion).

Glass transition temperature: 60–65°C.

Advantages: easy to print, a low melting temperature compared with ABS and polyamides (150–220°C), biocompatibility, a wide choice of colors, and many modified versions (higher strength, glow-in-the-dark, matt, wood-filled and so on).

Drawbacks: low heat resistance (it distorts above 60°C), brittleness, limited impact strength.

Typical uses: decorative models, prototypes, mock-ups, teaching models and souvenirs.

Post-processing: the material takes mechanical sanding well. Chemical smoothing calls for aggressive and hazardous solvents and is rarely used in practice.

ABS (acrylonitrile butadiene styrene) — the classic engineering plastic

An impact-resistant, heat-resistant amorphous thermoplastic, one of the most widely used materials for functional prototyping and end-use parts. ABS is built from three components: acrylonitrile, which gives impact and chemical resistance; butadiene, which gives toughness and strength; and styrene, which makes post-processing easier and adds rigidity

Temperatures: extruder 230–270°C. Build plate 90–110°C (heating is mandatory). For large parts an enclosed, temperature-controlled chamber is critical.

Glass transition temperature: about 105°C.

Advantages: high mechanical strength, heat resistance, good chemical resistance to oils and weak acids.

Drawbacks: considerable shrinkage on cooling, a tendency to warp, and the need for ventilation because toxic styrene vapors are released during printing. Sanding is harder than with PLA because the material is harder

Typical uses: instrument housings, functional prototypes, automotive components, parts that work at elevated temperatures.

Post-processing: the gold standard is chemical smoothing in acetone vapor, which gives an almost molded, glossy surface. Mechanical sanding is difficult.

ASA (acrylonitrile styrene acrylate) is used as well. It was developed as a replacement for ABS with substantially better resistance to ultraviolet light and weathering, while keeping the main mechanical properties of its predecessor.

PETG (polyethylene terephthalate glycol) — the all-round compromise

PETG combines a number of the advantages of PLA and ABS, offering high impact resistance, transparency, biocompatibility, recyclability and chemical resistance with a relatively straightforward printing process.

Temperatures: extruder 220–260°C. Build plate 70–85°C (heating is mandatory).

Glass transition temperature: 75–85°C.

Advantages: high impact resistance, chemical inertness, transparency, suitability for food contact (after appropriate treatment), low odor emission.

Drawbacks: shrinkage on cooling, difficulty in keeping the melt flow constant, which leaves a pronounced texture on printed parts, poor resistance to scratching and rubbing, problems with first-layer adhesion, sensitivity to UV light.

Typical uses: functional parts, tooling components, durable prototypes, containers, parts in contact with aggressive chemicals.

Post-processing: chemical polishing with the usual agents (acetone) is impossible. The main method is mechanical sanding. Paint adhesion may be lower than on PLA or ABS, so good primers are needed.

TPU/TPE (thermoplastic polyurethane / thermoplastic elastomers, Flex) — flexible materials.

These materials make it possible to print elastic, rubber-like objects over a wide range of hardness.

Temperatures: extruder 200–230°C. Build plate 40–60°C.

Advantages: high flexibility and stretch, shock absorption, low shrinkage, wear resistance, a wide service temperature range, resistance to oils and gasoline.

Drawbacks: low print accuracy, difficult printing because the filament is soft, the need for a direct drive extruder, strong adhesion to the build surface.

Typical uses: seals, flexible hinges, damping inserts, cases, prototypes of rubber parts.

Post-processing and painting: the usual sanding and painting methods do not apply. Painting calls for special flexible primers and paints that can deform together with the substrate. Any paint, however, reduces the flexibility of the model. Coloring the material in bulk before the filament is extruded is a common alternative.

Nylon (polyamide, PA) — high strength and wear resistance

Nylon belongs to the family of thermoplastic polyamides, which stand out for their exceptional tensile strength, abrasion resistance and good impact toughness. PA is further classified by chemical composition, in particular by the number of carbon atoms (n) — the most common PA (n) grades on the market are PA11, PA12 and PA6, which are used in FDM. PA6 is the most common; it is highly flexible and resistant to impact and abrasion.

Temperatures (PA6): extruder 240–270°C. Build plate 90–110°C. Drying the filament beforehand is critical.

Glass transition temperature: about 50-60°C (it keeps its mechanical properties up to 80-100°C).

Advantages: low price, high strength, excellent wear and sliding properties, a low coefficient of friction, biocompatibility, chemical stability, elasticity.

Drawbacks: high hygroscopicity (it takes up moisture from the air), considerable shrinkage; nylon has to be printed on a heated plate (around 80°C) because of adhesion problems and to keep it from absorbing moisture from the surroundings, which would affect print quality.

Typical uses: loaded functional parts: gears, plain bearings, tool housings, engineering prototypes.

Post-processing: the main difficulty is extremely poor adhesion of paints and adhesives caused by the low surface energy. The effective industrial coloring method is dyeing — immersing the part in a hot dye bath. At home the surface has to be prepared carefully and a specialized adhesion promoter used.

Photopolymer resins (for SLA/DLP/LCD)

These materials are liquid compositions (monomers, oligomers, photoinitiators) that polymerize under UV light and form a solid polymer. They are thermosetting materials.

Properties and advantages: the highest resolution and the smoothest surface in consumer 3D printing, isotropic mechanical properties, a wide range of specialized formulations (flexible, castable, biocompatible).

Advantages: good chemical and heat resistance, high bond strength, isotropy.

Drawbacks: dimensional inaccuracy, processing defects, reduced mechanical properties and incomplete curing, which may be caused by scattering of the ultraviolet light due to a mismatched refractive index.

Process notes: post-processing after printing is mandatory: washing in a solvent (usually isopropyl alcohol) to remove resin residue, followed by a final cure in a UV chamber to reach full strength.

Preparing for painting: although the surface is smooth to begin with, a primer coat has to be applied for the paint to adhere.

Specialty and industrial plastics (a brief overview)

Besides the materials above there are others that are used less often and, as a rule, by enthusiasts or in industry. Here is a short list of the most notable ones and of how they behave when painted:

Polypropylene (PP) — very flexible and chemically resistant, but extremely hard to print because of heavy shrinkage. Its main drawback is practically zero adhesion to paints and primers, which makes painting it at home an impossible task.

POM (polyacetal) — a high-strength engineering plastic with a low coefficient of friction, ideal for parts such as gears. Printing it involves considerable difficulty (very high shrinkage, the need for a heated chamber, poor bed adhesion), so it is rarely used.

Polycarbonate (PC) — a structural thermoplastic widely used in 3D printing for engineering tasks. It combines exceptional impact resistance ("unbreakable plastic") with high heat resistance and good optical clarity. Clean extrusion calls for high nozzle temperatures; the material shrinks and is hygroscopic.

PEEK — a member of the high-performance thermoplastics class, seen as a promising alternative to thermosetting composites in demanding industries. It is a semi-crystalline polymer with a unique combination of properties, which makes it a material for extreme engineering and special applications. It calls for extreme printing temperatures (nozzle above 360°C, heated chamber) and specialized equipment. It is not used at home. Painting is difficult because of the smooth surface.

Overall conclusion: these materials are meant for specific engineering tasks, not for decoration. Using them, let alone painting them, at hobby level is extremely limited or impossible.

Surface preparation before painting (post-processing methods)

3D printed parts have a characteristic layered texture: the layer lines are visible and curved surfaces can feel like orange peel. If the aim is a smooth, professional look, the surface has to be leveled before painting. There are two basic approaches: mechanical work (sanding, abrasive polishing) and chemical smoothing. A combination of the two is often used for the best result. Let us look at how to prepare a plastic model for painting.

Mechanical smoothing (sanding)

This is a universal method that suits practically every kind of plastic. It consists of cutting or grinding away the raised ridges of the layers with abrasives.

Method: sand in several stages, stepping up the grit each time. For example: start with P150–P200 paper to remove coarse artifacts, flash and support marks; then move on to P320 and P600; finish with P800–P1000 to smooth out the fine scratches. For the final pass use wet sanding (fine paper with water), which keeps the abrasive from clogging with dust and the plastic from overheating.

Sanding tips:

• sand across the layer lines for better leveling.
• for hard-to-reach places use needle files, small sanding sticks or sanding sponges.
• large flat surfaces can be leveled first with a scraper or a blade, shaving off the "comb" of the layers.
• avoid overheating the plastic through friction (this matters most with PLA) — use less pressure and longer strokes.
• always use protective equipment: a respirator against plastic dust, and gloves.

After sanding the surface is flatter but still matt, with fine scratches on it. These defects can be dealt with using a filler primer.

Filling gaps and using body filler

After sanding, a print may still show visible gaps left by the limits of the print path. To close them, use filling:

• small gaps and voids are filled with epoxy resin or a liquid filler.
• for larger defects use automotive body filler, which has to be sanded after it is applied.

Chemical smoothing (solvent treatment)

For some plastics there is a way to remove the layer lines without mechanical work — dissolving the top layer of the surface so that it flows out and levels the unevenness. This method does affect the dimensional accuracy of the object, however, because the amount of material removed cannot be controlled.

Where it applies: the method does not work with every plastic.

effective for: ABS and ASA. The classic approach is an acetone vapor bath in a closed container for 10–30 minutes, which gives the part a glossy look with the layers gone.

ineffective or dangerous for: PLA and PETG. PLA can react with dichloromethane, tetrahydrofuran or ethyl acetate, but these solvents are extremely toxic and flammable and the process is hard to control. PETG is chemically resistant to most household solvents.

Key points and safety:

• safety: acetone and other solvent vapors are flammable and explosive, and they are harmful to health. Work in a well-ventilated non-residential space, away from open flames and electronics, wearing gloves and a respirator.
• timing: too long in the vapor (especially with ABS) makes the part too soft, blurs fine detail and rounds off edges. The optimum is 10–20 minutes. Once it is taken out, the part has to cure for several hours.
• uneven treatment: vapor rises from the bottom up, so horizontal surfaces are treated more intensively. For an even result the part can be hung up or turned over from time to time.
• preparing for painting: chemical smoothing leaves a glossy surface with poor adhesion. It has to be scuff-sanded with very fine paper (P800–1000) before the primer goes on.

Combined approach (recommended for ABS/ASA): for the best result experienced modelers combine both methods: first coarse mechanical sanding, then chemical smoothing to remove the small defects and add gloss, after which the surface is lightly scuffed so that the paint will adhere.

Important warning: chemical treatment can dissolve adhesive and filler. Parts made of several pieces should be glued together after the treatment, or bonded with the same solvent (acetone for ABS, for instance, which welds the pieces together). Thin elements (under 1 mm) may distort.

Post-processing of parts printed by SLA (photopolymer resins)

Models printed on SLA/DLP printers go through a mandatory post-processing cycle:

• Washing: removing uncured resin residue with isopropyl alcohol (or another recommended solvent).
• Drying.
• Final UV cure: completing the polymerization and improving the mechanical properties in a UV chamber. As researchers note, UV and thermal curing are the most common methods of improving the degree of polymerization after SLA printing.
• Mechanical work (if needed): removing the support structures and wet sanding to get a smooth surface (particularly effective on parts with complex geometry).
• Finishing (optional): applying a coat of mineral oil on certain mechanical parts, although such a surface holds paint poorly.

Annealing (heat treatment) of PLA

Annealing is a heat post-processing method for PLA in which the part is heated above the glass transition temperature (60–65°C) but below the melting point (173–178°C). Done correctly, it recrystallizes the material, increasing its stiffness by about 25% and its strength by about 40% on average, and it can also raise the heat resistance slightly.

Priming and surface repair

Once the geometry of the part has been leveled with sandpaper and/or solvent, it is time to prepare the surface for painting. This stage covers degreasing, applying an adhesion promoter and dealing with small defects using a filler primer. Good preparation is the key to a durable, even paint finish.

The tools and materials for preparing a 3D print for painting: abrasive sheets in several grits, a mask or respirator, gloves, a degreaser and primers. Work in a well-ventilated space and protect your airways — sanding and spraying produce harmful particles and vapors. Do not touch the cleaned surface with bare hands, so as not to leave greasy marks that impair the adhesion of the finish.

Degreasing. After sanding, clean all the dust off the model (with a brush, compressed air or a soft cloth). Then degrease the surface with a dedicated product, for example the KUDO^® universal degreaser KU-9102. Automotive refinishing often uses a silicone remover such as the KUDO^® KU-9100 (aerosol) — it removes grease, dust and fingerprints. The special KUDO^® multipurpose anti-static cleaner KU-9103 can be used as well. Do not skip this step: even the slightest trace of oil or sweat from your hands can cause paint defects (fisheyes, delamination).

Adhesion promoter (plastic primer). Immediately after degreasing, without touching the surface, apply the adhesive activator — a special primer for plastics. It is essential above all on the "difficult" plastics (ABS, PETG, nylon, PP), but it does no harm on PLA either. An adhesion promoter is a clear composition that soaks into the top layer of the plastic and makes it capable of gripping the paint firmly. In our range this is the KUDO^® KU-6000, supplied as an aerosol. Spray one thin, even coat of the adhesive activator over the whole surface and let it dry (usually 5–10 minutes, see the instructions). During that time the primer reacts with the plastic, slightly softens its surface molecules and polymerizes itself, forming a "bridge" between the plastic and the coats that follow. Note that the adhesive activator has to go on very thinly — it does not level the surface, it only modifies its chemistry.

Filler primer (leveling). If the part still shows fine scratches, pinholes or other defects, a filler primer is worth using. It is a thick acrylic primer that is applied in a heavy coat and sanded once dry, filling the small imperfections. We recommend two options: the KUDO^® acrylic filler primer KU-220X (gray, high build) or the KUDO^® universal acrylic primer KU-210X. Both suit plastic, but on difficult plastics they are better applied over the adhesion promoter. Spray 1–2 coats of filler primer with flash-off in between: the first a mist coat, the second a full covering coat. Watch out for runs — hold the can about 20–30 cm away and move it evenly. The primer will close the sanding marks (the scratches left by the paper) and the small dips well.

If the sanding marks are too deep, you can use the KUDO^® spray putty KU-2242. It is, figuratively speaking, a filler primer squared: it can be applied in even thicker coats and masks defects that a filler primer can no longer hide. In every other respect the application and sanding technique is exactly the same as for a filler primer.

Let the filler dry completely (at least 2–3 hours, or better a full day if you have the time). Once it is dry, take fine sandpaper (P600–800) and sand the primed surface with light circular strokes. The aim is to take the primer off the high spots and leave it only in the scratches, arriving at a perfectly flat plane. Use side lighting or light wetting to check your work — the matt surface should come down evenly. Be careful on sharp edges and thin elements: they are easy to sand through to the plastic, and the primer then has to be applied again. This stage should end with a perfectly smooth, evenly matt surface with no visible flaws. It should feel like velvet, with no abrupt steps.

After sanding the primer, dust the part off once more before the finish coats go on. Wear gloves so as not to leave grease on the freshly sanded surface.

Preparing and painting a printed part

Choosing the paint and painting

Once the surface of the part is prepared and primed, you can move on to the main job — applying the color coat. Choosing a paint for plastic is a matter to take seriously: not every paint holds equally well on plastics. The best option for 3D printed parts is acrylic aerosol paint. It dries fast, is reasonably flexible and comes in a wide color range. Below we go through the order of the coats and the products we recommend.

Using the 2-in-1 primer-enamel. One distinctive product in our range is the KUDO^® primer-enamel for plastic KU-60XX. It is an acrylic material that combines the properties of a primer and a topcoat. It can go straight onto the plastic (although we prefer to put it over the adhesion promoter, to be sure) and it builds a durable coating in the chosen color. With the primer-enamel an object can be painted in the color you need in just 2–3 coats, with no separate primer and no separate topcoat. The result is a durable matt finish. If one of the standard KUDO^® KU-60XX colors suits you, no further painting is needed once the coats of this primer-paint are on — the part is already colored and protected. The black primer-enamel, for instance, is often used as the base coat under chrome or chameleon paints. And if you need an unusual shade that the KUDO^® KU-60XX range does not have, the primer-enamel can serve as a colored primer coat with a different paint applied over it.

Applying the color coat. Acrylic aerosol paints have proved the best finish coats on plastic. They give a bright color, spray evenly and dry fast. From our range, the following suit plastic: the KUDO^® RAL fast-drying acrylic paint KU-AXXXX (high-gloss and matt shades), the KUDO^® SATIN RAL silky matt acrylic paint KU-0AXXXX (for a more subdued sheen) and the KUDO^® pastel acrylic paint KU-A10x. Acrylic paints are applied over the primed surface in two or three coats until the color is uniform. Several thin coats are better than one thick one — that way you avoid runs and get an even result. Spray each following coat once the previous one has partly dried (after 5–10 minutes; see the instructions on the can for details). If you need to mask off different colors, use masking tape, but do not leave it on fresh paint for long (pull it off as early as you can). Alkyd products may be applied over a surface primed with an acrylic primer — universal and automotive alkyd paints, for example. Because of their limited flexibility, though, alkyd paints are not recommended, especially on flexible plastics.

Metallics and special finishes. To give a model a metallic shine (silver, gold, chrome and so on), use the metallic paints. In our range these are the KUDO^® acrylic paint lines SILVER GRAIN FINISH KU-10XXS, REFLECTIVE FINISH KU-10XXR and MIRROR FINISH KU-103X, plus the cellulose ester automotive base coats KU-4XXXXX. Each of these products contains a particular type of pigment (aluminum powder, pearl and so on) that produces its own effect, from a shimmering metallic to mirror chrome. Note: metallics go only onto a surface that has already been primed or painted. That is, the part is first covered with a white or gray primer, and the metallic is then sprayed over it in light coats. A metallic paint applied straight onto bare plastic without primer will very likely break up into patches, because the smooth metallic pigments adhere poorly. So follow the instructions exactly: a base coat of primer, then two or three coats of metallic with flash-off between them. Contrary to a widespread belief, not every metallic needs a coat of varnish, and not every metallic can take one. Varnish is mandatory only over automotive metallic base coats; SILVER GRAIN FINISH metallics do not have to be varnished, and REFLECTIVE FINISH and MIRROR FINISH must not be varnished at all — it will spoil the appearance.

For the most striking finishes there are special paints: the KUDO^® fluor effect paint KU-120X (bright neon colors), for example, or the KUDO^® chameleon paint KU-C267-x (which shifts through different shades with the viewing angle). Fluorescent paints go over a white primer or a white base, in thin coats, until the color reaches full neon brightness. The chameleon paint contains special pigments that show up on a dark substrate, so it is best applied over a black primer or a black primer-enamel. Both lines are acrylic, so they can be varnished on top to deepen the color and add protection.

Practical tip: let the coats of paint dry completely. An acrylic aerosol may be dry to touch in 10–15 minutes, but full curing takes several hours. If you plan to mask off and spray another color, wait at least overnight, otherwise the still-soft coat may wrinkle under the tape. With a multi-layer build (primer + paint + varnish) the total film thickness can be of the order of 100–200 microns, which swallows very fine detail — allow for this when you design the model (fine engraving and textures less than 0.2 mm deep can be filled in by the paint).

Alkyd products may be applied over a surface primed with an acrylic primer — the KUDO^® RAL alkyd spray paints KU-1XXX and the KUDO^® automotive paints KU-4XXXX, for example. Because of their limited flexibility, though, alkyd paints are not recommended, especially on flexible plastics.

To give plastic a particular texture — on interior or exterior car parts, for example — there is the special KUDO^® 1K textured plastic spray paint KU-62XX.

The finish coat: varnish

The final touch is a coat of clear varnish. It does two jobs: protection (it shields the paint from scratches, fading and moisture) and decoration (it sets the final finish — gloss or matt). Not every project needs varnishing: if you used our primer-enamel with its matt finish and you are happy with the way it looks, you can leave it as it is. In many cases, though, a coat of varnish gives the object a finished, factory-like look and an even sheen. In any case, read the instructions for the paint you intend to varnish carefully — it may turn out that this particular paint must not be varnished.

With our materials we recommend four KUDO^® 1K acrylic aerosol varnishes: the gloss universal KU-9002 and automotive KU-9010 (both give a mirror shine), and the matt universal KU-9004 and automotive KU-9010M (both give a matt, velvety finish).

Varnish is applied much like paint: in thin, even coats. The first coat is a very light mist coat that creates a semi-matt base. The second is a wet coat, sprayed to an even shine. A gloss varnish flows out evenly if you apply enough for the surface to become uniformly wet without running. The key word is enough. The "thicker is better" principle does not work here: an excessively thick coat, even if it does not run, will take a very long time to dry and may never dry through completely. A matt varnish must not be applied too thickly for another reason as well — it can go milky. Two coats are usually enough.

After varnishing it is best to leave the object alone for a while. The varnish may be dry to touch in an hour or two, but it needs time to build strength. If you can, put the painted object in a warm, dry place for 5–7 days — the finish will become harder and tougher.

Varnishing is particularly recommended for models that will be handled often or kept outdoors. Clear varnish contains UV filters that protect the color from fading. A gloss varnish also adds depth to metallics and chameleons, making the effect more pronounced, while a matt varnish removes the highlights by scattering the light evenly across the surface.

Conclusion

Painting 3D printed parts is a craft in its own right, combining knowledge of materials with painting skills. We have looked at how the properties of different plastics affect the choice of treatment and paint: some (ABS/ASA) allow chemical smoothing, others (PLA, PETG) call for patience with sandpaper and primer, and a third group (nylon, PP) takes paint reluctantly unless special measures are used. The key points worth repeating:

• Careful preparation is 90% of the result. If you have leveled and primed the model well, painting it will be easier and the result will look professional. Do not begrudge the time spent on sanding and priming.
• Take the material into account. Knowing what the part is printed from tells you the right approach: ABS can be smoothed with acetone, PLA is better sanded, TPU is better left unpainted where possible, and so on. Do not hesitate to experiment on test pieces before you work on a large model.
• Use the right coatings. Acrylic primers and paints suit plastics best — they give good adhesion and a flexible film. Always use an adhesion promoter for plastic if you want the finish to last: it is inexpensive, and the difference it makes is enormous.
• Safety first. Work in a ventilated room or a spray booth. Wear a respirator when spraying paint and gloves when working with solvents. Protect your eyes and skin. Remember that aerosol cans and their vapors are flammable.
• Practice and care. It is better to practice on a scrap piece than to ruin the final part. Follow the instructions printed on every can, and do not try to speed the process up with a heat gun — you can warp the plastic. Patience, and patience again: that is what gives your printed models an excellent finish.

We hope this guide has helped you work out the finer points of finishing and painting 3D printed parts. DIY technology today can reach a quality comparable to industrial work — you only need the right materials and the right methods. Good luck with your projects, and may your layers be even, in printing as well as in painting.

Quick application chart