Primer: a base under paint on a 3D print

What primer is good for

Primer does three things: it unifies the colour of the surface, so as little as possible shows through the final coat; it creates a rougher layer that paint grips better; and, in the thicker type (filler primer), it fills small grooves between layers. There is a general overview on Wikipedia.

With PLA and PETG it matters mostly for PETG, which has a smooth surface and paint holds worse on it without a base.

How to use it

• Type. Look for a primer made for plastics. A spray filler primer is the most practical.
• Preparation. Sand the surface and degrease it. See sanding.
• Thin coats. Two to three thin sprays from 20 to 30 cm with a rest between them. A thick spray runs and smooths away detail.
• Check. Once it dries, the primer shows every flaw that was hidden beneath it. These are the cue for more filling and sanding.

Limitations

Primer always adds a tenth of a millimetre of thickness, so it is not good for precise parts, threads and snap-fit joints. If a part is exceptionally detailed, a thick filler will hide fine detail. Solvents in sprays can damage thin walls, so test on a hidden spot. Wear a respirator and ventilate while you work. We do not offer this finishing as a service, parts arrive as the printer prints them.

On a single-colour part that does not need to be perfect, primer is not necessary. It is useful where you want an even, uniformly painted surface, for example on figurines, models or prototypes for presentation.

When ordering

The surface of a finished part will have visible layers at 0.2 mm and possibly marks from support removal. If you plan to paint the part, expect that preparing the surface takes time and a few rounds of sanding and spraying.