Sanding 3D Prints

What sanding changes

Sanding is not only cosmetic. It removes material, so every pass makes a part slightly smaller and rounds its edges. That is fine on a display piece and a problem on a part that has to fit something:

• Holes get bigger and pegs get smaller. Leave mating faces, snap fits and bearing seats alone, or sand them deliberately and test the fit as you go. See Fits and Tolerances.
• Fine detail goes first. Polymaker suggests starting at a grit that removes layer lines "without distorting details", and warns against high-speed tools on thin PLA walls.
• Power sanders remove material fast. Formlabs warns that a sander left stationary can quickly wear down an area, and that power sanding can destroy small features.
• Sanding through to infill can happen on thin FDM walls. Prusa's post-processing article notes that a break-through into infill can be covered with putty rather than reprinting; see Filling and Priming.

Dust: follow the material's SDS

Sanding turns a solid part into fine dust you can breathe. The hazard depends on the material, its colorants and fillers, so the SDS for your filament or resin is the document that sets your protection. What the sheets and makers consulted here say:

• Filament. UltiMaker's PLA SDS says that if prints undergo post-processing that causes dust, you should reassess whether that may lead to significant exposure and apply measures such as additional ventilation, air extraction or face masks, depending on the level of exposure. It raises this for colorants such as titanium dioxide: the sheet says the exposure risk during printing is negligible because the pigment is bound in the plastic, while California Proposition 65 lists airborne particles of it of respirable size as a carcinogen.
• Resin. Formlabs' support-mark guide says to wear a face mask, wear gloves or wash your hands after sanding, and recommends wet sanding to reduce dust. Resin has its own lesson: Finishing Resin Prints.
• Fillers and primers you sand later have their own SDS too. See Filling and Priming.

Practical habits that follow from those sheets: sand wet where you can, sand in a ventilated space, clean up with a damp cloth rather than blowing dust around, and wash your hands before eating. Your shop layout is covered in Setting Up a Safe Workspace.

Grit order

Each grit's job is to remove the scratches the previous grit left. Skipping too far up the scale leaves deep scratches that the finer paper cannot reach. Prusa's article warns against "unnecessarily large scratches" that then need extra fixing.

Stage	What makers suggest	Notes
First pass on FDM layer lines	220 grit for most materials such as PLA; 240 to 320 for ABS (Polymaker) Starting point	Polymaker says lower grits such as 180 can be used on hard plastics but risk heat-related deformation
Rough shaping of filled or large parts	P100 (Prusa's article, author's preference) Starting point	Used on filler and large smooth shapes, not on fine detail
Fine finish	800 to 2000 grit for polished surfaces (Polymaker) Starting point	Wet sanding with 400 grit and finer, per Polymaker
Between primer coats	P400, wet or dry (Prusa's article) Starting point	See Filling and Priming
Resin support marks	150, 220, 400, 800, 1000, 2000 (Formlabs) Starting point	See Finishing Resin Prints

Grit numbers come from more than one scale. Prusa's article uses P-grades; the other sources give plain numbers. Read the back of your paper and treat all of these as starting points, not exact equivalents.

Heat: the PLA problem

Friction heats the surface, and Polymaker warns that coarse grits and power tools risk heat-related deformation. Its advice:

• On power sanders, avoid prolonged contact in one spot to prevent heat buildup, which it calls critical for PLA.
• Rotary tools are useful in intricate areas but need careful handling to avoid melting or gouging.
• Wet sanding reduces both dust and heat.

Polymaker notes that ABS sands more easily than PLA, and that some filaments are formulated for easier sanding. For material-specific notes see PLA and ABS and ASA.

Wet sanding

Polymaker recommends wet sanding with 400 grit and finer Starting point because it reduces dust and heat, and Formlabs recommends it on resin to reduce dust, using water or mineral oil. Use paper rated for wet use, keep the surface wet so the dust leaves as a slurry, wipe the slurry up with a damp cloth, and let the part dry fully before priming or gluing. As a precaution, keep that slurry and its rinse water out of the drain and dispose of it with the dust.

On resin parts, treat the slurry and rinse water as resin waste, and never pour them down a drain; see Resin Waste and Disposal.

Technique

• Vary the direction. Formlabs suggests random motions when wet sanding seams, so you do not create a grain.
• Sand lightly after each filler layer dries, as Formlabs suggests for seams on large assembled prints.
• Stop when the next step can hide the rest. If the part will be primed, a filler primer fills fine scratches; see Filling and Priming.