Sanding 3D Prints: FDM and Resin

Sanding is the foundation of all physical post-processing. Whether you are preparing a surface for paint, trying to eliminate layer lines, or producing a smooth display surface, the process is the same: start coarse enough to address the defect, work progressively finer, and do not skip steps.

Grit Progression

Each grit removes the scratches left by the previous grit and replaces them with finer ones. Skipping too many grits means the fine paper tries to remove scratches that are too deep for it to address efficiently — you spend much longer at a finer grit, and the result is often not clean.

Standard progression for FDM parts:

120–180 → 240 → 320 → 400 → 600 (primer ready)

For display quality: continue to 800 → 1000 → 1500 → 2000

Standard progression for resin parts:

400 → 600 → 800 → 1000 (usually paint-ready at this point)

Resin comes off the printer smoother than FDM and needs less aggressive starting grit.

Wet vs Dry Sanding

Wet sanding — using water or a lubricant with fine grits — produces better results above 400 grit for most materials. The water clears the sanding residue continuously, keeps the paper cutting cleanly, and produces a finer finish than dry sanding at the same grit.

Use a bucket of water with a drop of dish soap. Dip the paper frequently and wipe the part with a cloth between passes to assess progress.

FDM Layer Lines

Layer lines are the ridges left by the FDM deposition process. At a 0.2mm layer height they are visible but not deep. Sanding through them requires removing approximately 0.1mm of material from the high points of each ridge, which takes meaningful time with coarse paper.

An alternative for flat or low-curvature surfaces is filling the layer lines with spot putty or glazing compound (automotive filler) before sanding. Apply a thin coat, let dry, sand flush. This can significantly reduce total sanding time on flat surfaces.

Curved surfaces and intricate features are better sanded directly — filler tends to collect in recesses and obscure detail.

Protecting Detail

Sanding rounds edges and softens fine detail. For parts where sharp edges and fine detail must be preserved, sand only the flat and curved surfaces and avoid edges. Use small sanding sticks (craft supplier wooden sticks with paper wrapped around them) to get into recesses while preserving the surrounding geometry.

Material-Specific Notes

PLA: Sands easily but can smear if the paper glazes. Keep paper clean.

PETG: Slightly rubbery and can be harder to sand to a smooth finish. Higher pressure and fresh paper produce better results.

Resin: Sands very cleanly. Creates fine dust — wear a dust mask as resin dust is a respiratory irritant.

TPU / Flexible materials: Do not sand — the material compresses and deforms under the paper rather than removing material.