Sanding, priming and painting 3D prints: grit order, acrylics and clear coat

A printed part that gets sanded, primed and painted in the right order can look like a finished product. The order is: sand away layer lines, prime to reveal flaws, sand again, paint in thin coats, then clear coat. Most failures come from skipping a step or rushing paint that is not dry.

This follows on from Smooth surfaces on 3D prints, which covers slicer-side smoothing and vapor smoothing. Here we go into the hands-on part: sanding, primer, paint and clear coat.

Plan for it when you print

Sanding removes material. If you know a part will be painted, use at least 3 perimeters so you don’t sand through to the infill. Thinner layers, around 0.12–0.16 mm, cut sanding time a lot. For parts with several colours, printing the pieces separately and painting them before assembly is easier than masking (see Joining 3D prints).

Grit sequence

Step	Grit	What it does
1. Remove layer lines and support marks	220 or 320	Sand in small circles, not along the layers
2. Remove the coarse scratches	400 or 600	Even matte surface, no deep scratches
3. After the first primer coat	finer than 600	Only where the primer shows it still isn’t smooth
4. Between paint coats (for a high gloss)	finest you have, wet	Knock off dust nibs

These grits follow Formlabs’ guide to painting printed parts and match what model painters commonly use.

Wet sanding means waterproof paper dipped in water. The water carries debris away, the paper doesn’t clog, and dust stays down. PLA should only be sanded wet, per Prusa, because it softens at low temperatures and friction heat makes the surface gummy instead of smooth. Don’t use a high-speed power sander on PLA.

When you finish, wash the part with water and a soft brush and let it dry completely before using a tack cloth. The wax in a tack cloth doesn’t work on a wet surface.

Primer: find the flaws before paint

Primer does two jobs: it gives paint something to grip, and it reveals flaws you can’t see on single-colour plastic. A mid-grey primer makes layer lines and sanding scratches stand out.

• Spray primer gives a fast, even coat over the whole part.
• Brush-on primer is for small touch-ups.
• Filler primer is thick enough to fill shallow layer lines, but the thicker it goes on, the more detail it buries.

Order: one thin coat, let it dry per the label, inspect, fill deep marks with a thin skim of putty, sand finer than 600, clean, then spray the final coat. Pick a primer labelled for plastic and, where you can, from the same system as the paint.

Acrylic paint: several thin coats beat one thick one

Acrylics are the most common choice for printed parts, applied by spray can, brush or airbrush.

1. Undercoat: 2–4 thin coats. The primer can show through the first one; don’t rush to full coverage.
2. Once a coat is opaque, let it rest about 20–30 minutes.
3. Topcoat, if you want one: 1–2 light coats.
4. Clear coat: 1–2 thin, even coats to protect the paint from fingerprints and scratches. Gloss, satin or matte to suit the part.

For spray cans, take the spraying distance and recoat time from that product’s can. Several light passes with a pause beat one long wet pass.

Paint that feels dry is not fully cured. Let the part rest at least a week before handling it hard, and handle it gently with gloves until then.

Worked example: painting schedule for a 12 cm figure

Assume PLA at 0.16 mm layers with 3 perimeters. Take drying times from your primer and paint labels; the numbers below are for planning.

Day	Step	Rough time
Day 1 morning	Wet sand 320 → 600, wash, let dry	45 min + drying
Day 1 afternoon	First primer coat, dry per label, inspect, putty	30 min + drying
Day 2	Sand finer than 600, wash, final primer coat	40 min + drying
Day 3	3 undercoats, about 20–30 min apart	about 1.5 h
Day 4	2 clear coats	30 min + drying
Day 11	7 days after clear coat, normal handling	—

Hands-on time is under 5 hours. The rest is waiting. Skip the waiting and you get wrinkled paint, sticky spots or a fingerprint sealed in forever.

PLA and heat

Prusament rates PLA’s temperature resistance at about 55 °C and says it starts losing strength above 60 °C. So:

• Never oven-dry paint, and keep hair dryers and heat guns away.
• Don’t leave it in direct sun or in a car. Thin bases and fins will warp.
• If paint is still tacky after the label time, give it more time instead of adding heat.

PETG and ASA handle more heat, but the rule is the same: dry in the shade, per the paint label.

Safety

• Sanding dust: wear a dust mask. Wet sanding cuts dust a lot.
• Spray paint, primer, clear coat: use a proper respirator (Formlabs recommends a NIOSH-approved one) and work with good ventilation or outdoors.
• Nitrile gloves keep paint off your hands and fingerprints off the part.
• Many spray cans are flammable. Read the warnings on the can and keep away from flames and heaters.

Common mistakes

• Dry power-sanding PLA. The surface turns into gummy lumps that have to be cut off.
• Skipping primer. Paint grips poorly, and layer lines you thought were gone reappear under gloss.
• Spraying thick for coverage in one coat. Runs and pooled edges.
• Tack cloth on a wet part. Dust stays, cloth is ruined.
• Handling before it cures. Fingerprints sealed into the clear coat.
• Heat-drying. A beautifully painted PLA part, now warped.

Next: Multi-colour printing options

Sources

• Guide to painting 3D printed parts: priming — Formlabs
• PLA — Prusa Knowledge Base
• Prusament PLA — material page
• ASA — Prusa Knowledge Base