A raw FDM print tells you it was printed. The layer lines are visible, the surface is slightly waxy, and any support structure leaves a texture where it was snapped off. For a jig or a hidden bracket, none of that matters. For a part you're going to look at every day, it's the difference between "3D printed" and "made".
Finishing isn't magic — it's a sequence, and doing the steps in order saves you hours of rework.
Why finishing matters beyond looks
Finishing isn't only cosmetic. Filler primer seals the microscopic porosity between layers, which matters on a part that lives outside and takes on water. A sealed, painted surface is also easier to clean and holds up better to UV than bare filament. And on a reproduction part, the finish is half of what makes it convincing against the original.
The order of operations
The sequence that works for us, for a cosmetic part:
- Remove supports and trim any brim or stringing with a sharp blade.
- Sand — start coarse, work fine.
- Fill low spots and layer lines with filler or filler primer.
- Prime the whole part so paint has something to key to.
- Paint in thin coats.
- Clear coat if the part is exterior.
Skip a step and you'll usually pay for it in the next one: paint doesn't hide layer lines, it highlights them.
Sanding layer lines
Start around 120–180 grit to knock the ridges down, then step through 240, 320 and 400. Don't press hard — let the paper do the work and keep the surface cool. Heat is the enemy: sanding too aggressively softens the plastic and smears it instead of cutting it. PETG is more prone to this than PLA or ASA, which is one reason material choice matters before you ever think about paint.
For tight inside corners a flexible sanding sponge or a rolled strip of paper beats your fingertips.
Primer and filler
Filler primer is the shortcut that makes FDM parts look moulded. It's a high-build primer that fills the fine lines left after sanding, then gets sanded back itself. Two or three coats, sanding lightly between, will take a part from "definitely printed" to smooth.
For deeper defects — a support scar, a seam, a small void — use a spot filler first, then primer over the whole surface. Give it a proper flash-off time between coats; rushing primer is the most common reason a finish fails a few weeks later.
Paint and clear coat
Thin coats, more of them. Two light passes beat one heavy pass every time — heavy paint runs, pools in corners and takes forever to cure. For a flexible or exterior part, use a paint and clear coat rated for plastic, and let the clear coat fully cure before the part gets wet or mounted.
What you can safely skip
Not every part earns the full treatment. A jack point cover that's hidden under the sill gets sanded just enough to be handled cleanly and nothing else. A bracket nobody sees ships raw. Finishing should match where the part lives, not be applied uniformly because it's in the checklist.
If the idea of sanding a whole set of parts yourself doesn't appeal, tell us what you need — we can finish parts in-house, from a light sand to a painted, clear-coated part ready to bolt on.
