Priming and Painting 3D Printed Parts

Paint on plastic requires two things to last: mechanical adhesion from surface texture and chemical compatibility between the paint and substrate. Primer addresses both — it keys into the surface texture, provides a chemically compatible base for topcoat adhesion, and fills minor surface defects. Skipping it produces paint that peels, cracks, or fails adhesion tests.

Choosing the Right Primer

Filler primer (grey or white): The standard choice for most 3D printing applications. Available as rattle cans or aerosol. Higher solids content than standard primer fills minor surface imperfections and builds a smooth base. Grey filler primer is the most common for subsequent dark colours; white for lighter topcoats.

Adhesion promoter + primer: For flexible materials (TPU, flexible resin), standard primer may crack as the part flexes. Use a flexible adhesion promoter first, followed by a flexible primer formulation.

Self-etching primer: For metal-painted resin props or parts with metal hardware, self-etching primer bonds chemically to bare metal. Necessary if painting hardware inserts or metal armatures incorporated into a print.

Application Technique

Aerosol/rattle can: Hold 20–30cm from the surface. Apply in slow, even passes, slightly overlapping each pass. Never stop moving while spraying. Each coat should look just slightly wet — if it looks dry, it is too light; if it runs or sags, it is too heavy. Let each coat dry to touch (15–30 minutes) before applying the next.

Brush application: Mix primer to slightly thinner consistency than for spray. Apply with a good quality flat brush in smooth, overlapping strokes. Brush primer is more controllable in recesses and corners but leaves visible brush marks that require sanding between coats.

Between-Coat Sanding

After the primer is fully cured (24 hours for most rattle-can primers, not just touch-dry), lightly sand with 400–600 grit to remove any dust nibs, orange peel texture, or runs. A single light pass is sufficient — you are levelling not stripping. Wipe dust clean with a tack cloth or damp cloth before the next coat.

Topcoat Application

The same thin-coat technique applies to topcoats. Two or three thin coats produce a more durable finish than one thick coat. Allow each coat to dry to touch before the next.

For acrylic hobby paints (Tamiya, Vallejo, Citadel): thin to approximately milk consistency and apply in multiple thin passes.

For automotive rattle cans: follow the coat time instructions on the can. Most automotive paints require a specific window (often 10–30 minutes) for the next coat before the chemistry changes.

Sealing and Varnishing

For display pieces and props, a clear varnish or lacquer topcoat protects the paint layer and controls final sheen. Matt varnish for realistic finishes, semi-gloss for consumer product aesthetics, gloss for high-sheen applications. Apply over fully cured paint — not over paint that is still gassing off.

Painting Flexible Parts

Flexible parts (TPU, flexible resin) require flex additives in the paint or flexible-specific paints. Standard acrylic paint will crack on flexing parts. Add a flex agent to your topcoat at approximately 5–10% by volume, or use paints specifically formulated for flexible substrates.