How to spray paint rusty metal. Choosing the materials

How to spray paint rusty metal. Choosing the materials

A part covered with rust often has to be painted. We have tried to explain in as much detail as possible how it is done and which materials are used for it.

Surface preparation - neutralizing the corrosion

Careful surface preparation is what makes a paint film last. Preparation takes most of the time, which can come as a surprise to anyone who meets the job for the first time. Even light corrosion has to be removed. The analogy with tooth decay fits here: fill a tooth without removing the decay, and the tooth keeps falling apart fast. So rust has to be fought by every means available. Loose rust must be removed with a wire brush, after which the surface is sanded with sandpaper. From there you can choose one of three routes

The first: treat the surface with the rust converting primer KUDO® KU‑2601. Apply it in 2‑3 coats with a few minutes of intercoat drying and let it work for 24 hours. It does not have to be washed off; after 24 hours any type of primer or paint can be applied over it.

The second: treating the surface with a rust converter is impossible or very difficult. In that case the direct-to-rust primer-enamel KUDO® KU‑31XXXX, KUB-31xx-08 (900 ml) or the hammered finish paint KUDO® KU‑30XX is applied straight away. Under such conditions this is effective enough.

The third: treat the surface with a phosphoric acid rust converter — the liquid KERRY® KR‑540 or the gel KERRY® KR‑240, for example. The gel is easier to use on vertical surfaces because it does not run off. Isolated spots of corrosion are best treated with the rust converter in a bottle with an applicator brush KUDO® KU‑70005. The rust converter needs some 30‑40 minutes to work, after which it has to be washed off with water; dry the surface immediately and apply primer or paint to it. A surface treated with a rust converter must not be left without a protective coating, or the metal will start to rust again.

Applying the primer

Metal that has been cleaned of dirt, degreased and treated with a rust converter has to be primed. If the surface is smooth, scuff it with fine sandpaper or an abrasive pad before the primer goes on. Some paints — the wheel paint KUDO® KU‑520X and the brake caliper paint KUDO® KU‑521X, for instance — can be applied straight onto metal, but over a primer the result is still better. Primers do several jobs: they improve the corrosion resistance of the coating, improve the adhesion of the paint to the surface, hide minor defects of the substrate and make the topcoat smooth, make the color of the topcoat deeper and brighter, and form a cushioning layer that makes the coating more resistant to small stone chips. So which primer is for what?

The zinc rich primer KUDO® KU‑2301 conducts current well and can be used as a weld-through primer. The zinc in it also gives the metal additional galvanic protection. A zinc rich primer is of course applied straight onto the metal.

The alkyd universal primer KUDO® KU‑200X belongs under alkyd paints, and the paint is best applied over the primer wet-on-wet, as soon as the primer is dry to touch.

For repairing local damage the primer in a bottle with an applicator brush KUDO® KU‑70006 is convenient.

The acrylic universal primer KUDO® KU‑210X and the acrylic filler primer KUDO® KU‑220X work well as a secondary primer over an acid etch primer, a zinc rich primer or a rust converting primer.

Acrylic primers are good for hiding minor surface defects: they can be applied in a thick coat and then sanded, bringing the surface to a perfectly flat and smooth state. Acrylic paints, alkyd paints and a nitrocellulose metallic base coat can all be applied over them. The acrylic primer must be fully dry before the paint goes on.

Applying the paint – painting

Metal surfaces can be painted with acrylic, alkyd and cellulose-based paints (the automotive metallic base coat).

Acrylic aerosol paints split into solid colors (non-metallics) and metallics. The non-metallics in the KUDO® range are the satin SATIN RAL and the universal fast-drying RAL paints, high gloss and matt. SATIN RAL is very pleasant to the touch, it creates a silky feel.

The acrylic metallics SILVER GRAIN FINISH, REFLECTIVE FINISH and MIRROR FINISH differ in the type of pigment (more about pigment types here), but all of them go onto a primed surface only. The cellulose-based automotive metallic paint is also applied over a primer.

Alkyd aerosol paints can in principle be applied straight onto metal, but on a properly primed surface the appearance is better and the coating is harder and lasts longer. Alkyd paints behave in their own way during application, drying and curing, so we recommend reading about them here first.

Heat-resistant paints KUDO® KU‑500X deserve a separate mention. They are applied straight onto metal simply because there is no primer that would survive heating to 400‑800 °C. Two coats of heat-resistant paint are enough. After application the paint must be heat cured. How this is done is described in detail on the can and here.

Topcoat – applying the clear coat

A clear coat over the paint is mandatory only for a cellulose-based automotive metallic – in that case it cannot be skipped. The best choice is the 1K acrylic automotive KU‑9010; the universal acrylic KU‑9002 and KU‑9004 (gloss and matt) can also be used.

Matt acrylic paints can be cleared for extra gloss and additional protection, but this is not mandatory.

Alkyd paint should not be cleared – it gives enough gloss on its own and is harder than an acrylic clear coat.

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