Painting and Finishing 3D Prints

Every FDM part comes off the machine with visible layer lines. That is not a fault to be dialled out; it is what the process is. You can get a printed part to look moulded, but the work is in the finishing, not the printing — and the finishing is where all the time goes.

What you get before any finishing

• Vertical walls — fine, even ridges. On a small layer height they are subtle. Run a thumbnail across and you feel them.
• The top surface — the best face on the part, near-smooth.
• The bottom — a mirror of the print bed. A textured bed gives a nice even texture; a smooth one gives near-gloss.
• Shallow sloped faces — the worst. Stair-stepping is at its most visible when a surface is only slightly off horizontal. Layer height moves this a lot; see choosing layer height.
• Support contact faces — rough and dimpled, always. Where they land is an orientation decision, covered in supports and overhangs.

For a bracket in a cupboard, that is the end of the story. For anything on show, read on.

The honest process for a painted finish

There is no shortcut. The sequence is:

1. Remove supports and clean up. Snips, a knife, a deburring tool.
2. Sand. Start around 120-180 grit to knock the layer lines down, work up through 240, 400, 600. Wet sanding from 400 up cuts faster and stops the paper clogging.
3. Filler primer. This is the step that does the actual work. High-build automotive filler primer floods the remaining valleys. Two or three coats.
4. Sand the primer back, 400 then 600. You will sand through in places and see the plastic. That is normal.
5. Repeat 3 and 4 until the surface reads flat under a raking light. Twice is typical. On a big part, more.
6. Colour coat, several thin passes rather than one thick one.
7. Clear coat if you want gloss or protection.

That is hours of hand work per part, not minutes. It is the single most underestimated cost in 3D printing, and it scales with quantity in a way printing does not — ten painted parts is ten times the labour, whereas ten prints share a setup.

Material makes a real difference

• PLA sands beautifully. Stiff, no melting or gumming. The best material for a painted part.
• PETG is a nuisance. It is tough and slightly rubbery, so it gums the paper and tears rather than powders. Doable, slower.
• ABS/ASA sand well and take paint well. They also respond to a vapour smoothing process, which is a genuinely different route — but it needs an extraction setup and it softens fine detail.
• TPU effectively cannot be sanded or painted. Paint cracks off a flexible part. Choose the colour instead — see what is TPU.
• Carbon-fibre-filled grades come out with a pleasant matt texture that hides layer lines better than any plain plastic, straight off the machine.

Design for finishing

• Put the cosmetic face vertical or on top. Never against supports.
• Avoid shallow slopes on show faces. Steeper or flatter both look better than 15°.
• Radius sharp edges — paint pulls thin on a knife edge and reveals it.
• Split a part so each visible face prints well, then bond and fill the seam. Often better than one part with one bad face.

What we do and do not supply

Straight answer: we print, we do not run a paint shop. We remove supports, clean up the part properly and orient it so the good faces land where you need them. We will tell you before printing which face will carry the support marks and which will be the best surface.

We do not sand parts to a show finish or spray them. Painting done properly needs a dust-free spray environment and a lot of hours, and buying that from us would not be good value when the work is hand labour either way. Colour choice at print time is free; ask about it when you get an estimate, and see the materials we print.

Where FDM is the wrong process

• You need a flawless cosmetic surface and you are not doing the finishing. A printed part is not a moulded part out of the box. If nobody is going to sand it, do not expect it to look sanded.
• A high-gloss show surface on a complex organic shape. The finishing labour will dwarf the print. Consider whether the part wants moulding — see 3D printing vs injection moulding.
• Fine miniatures needing crisp painted detail. Resin is the right process. We do not offer it, and we would rather say so.
• Painted flexible parts. Physics is against you.