Sanding prints: how to get a smooth surface

Why sand

Parts from an FDM printer are built from layers that you can see and feel on the side. Sanding reduces them and prepares the surface for further steps, such as filling and painting. The basic principle, gradually refining a surface with an abrasive, is described on Wikipedia.

We do not sand parts, they arrive as printed, with visible layers. Below is the procedure for those who want to finish them at home.

How to do it

• Grit. Start with a coarser paper, about 120 to 180, and work up through 240, 400 and 600. Each step removes the marks of the previous one.
• Wet. With fine paper and water the surface heats up less. Fast dry sanding makes PLA soften and the paper clogs.
• Slowly and in circles. Do not press hard, plastic heats up from friction. PETG is softer and smears more than PLA, so it sands worse.
• Protection. Sanding dust is fine, wear a mask or respirator, especially when sanding dry.

What sanding changes

You are removing material from the surface, and each pass takes off tenths of a millimetre. On a thin wall or a precise joint with a 0.2 mm clearance, that can be the difference between a fit and a loose part. Details, text and edges get rounded.

If you know you will sand a part, design it 0.2 to 0.4 mm bigger on the surfaces that will be finished, and leave thicker walls. In the calculator choose higher strength for thin pieces. We do not do this finishing as a service, parts arrive the way the printer prints them.

With PLA a few minutes is enough on a small part. PETG is softer and smears more, the paper clogs on it, so add water and sand with less pressure. On larger surfaces use a sanding block so you do not create waves.

Order of steps

A typical procedure for a smooth finish is sanding, filler, sanding again, primer and finally paint. Each step is done in thin coats. See also filler and putty.