3D Printing Painting and Surface Finishes

Paint changes colour, not shape. Whatever the surface looks like under the primer is what the finished part looks like, so most of the post-processing happens before any colour goes on. How much of it a part needs comes down to how it was printed: a filament print, made by fused deposition modeling (FDM), needs filling and sanding first, while a resin print, made by stereolithography (SLA), comes off the printer nearly ready.

Still choosing between coats? Our guide to 3D print finishing compares them. This one is about getting a printed part ready for one.

Fused Deposition Modeling (FDM)

Filament printing builds a part from lines of melted plastic, so its surface is a series of fine ridges. Paint does not fill them. A thin coat follows whatever it lands on, which means spray painting a raw filament print gives you a coloured part with the layer lines still in it.

So the ridges come out first:

1. Fill. Putty goes into the layer valleys and any marks the supports left behind.
2. Sand. The part is sanded with a firm block through 220, 400 and 600 grit until it feels even under a fingertip.
3. Prime. A primer coat shows up anything the sanding missed, and gives the paint something to grip.
4. Paint. Two or three thin coats, sanded lightly between them, then a clear coat for sheen and protection. Faces that must stay raw are masked first, though a little overspray can still reach them.

That prep is skilled hand work done at our Singapore studio, and it is most of what painting a filament part costs. It also has a ceiling: sanding softens layer lines a great deal, but a sanded filament print is still a filament print. What filling and sanding can and cannot do shows where that ceiling sits.

Stereolithography (SLA)

Resin printing cures each layer with light at a much finer scale, so a resin part comes off the printer already smooth. It needs a light sand where the supports touched, a primer coat, and very little else before it takes paint.

That smooth start matters most for the two finishes that show every flaw beneath them:

• Airbrushing. An airbrush lays colour down finely enough for gradients, shading and weathering, which is why it is the finish for figurines and detailed models. Fine colour only reads well over a smooth surface.
• Electroplating. A real layer of chrome, gold, copper or brass over the print. Plating magnifies every scratch under it, so it suits resin best. Electroplating SLA prints covers how it works, and metal finishes compares it with metallic paint.

Both can go on a filament part too, but only after the full fill-and-sand routine above has made its surface smooth enough.

Painting big pieces

A part too big for one print is printed in segments and joined, and the finishing is what makes it read as one piece rather than a stack of parts. On a large display piece that is hand work: each segment is finished so the colours carry across the joins.

Pick a colour from our spray paint colour catalogue, or ask us to recommend a finish for your part.