Why PETG sands differently than PLA
PLA is rigid and brittle. It cuts cleanly under abrasive pressure. PETG has more elasticity and a lower friction melt threshold, which means heat from sanding can smear the surface rather than remove material. The result is a milky, cloudy layer that looks worse than the layer lines you started with.
Start finer than you think
On PLA, starting at 120 or 150 grit is often fine for heavy layer lines. On PETG, starting that coarse introduces deep scratches that are difficult to remove without generating more heat. Start at 220 or 240 and work slowly. If the layer lines are raised and stubborn, use short passes and check the surface every few seconds rather than pushing through.
Keep the surface cool at all times
Heat is the main enemy when sanding PETG. Dry sanding with sustained pressure will soften the plastic within seconds. Use light strokes, lift the sandpaper often, and let the part cool between passes. A damp cloth nearby to cool the surface periodically can help on larger flat areas.
Wet sanding is the better choice for most stages
Moving to wet sanding as early as 320 grit will significantly reduce gumming. Water acts as a lubricant and heat sink, letting the abrasive cut cleanly without smearing. Wet sanding PETG from 400 upward produces a noticeably cleaner surface than dry sanding at the same grit. Rinse the surface and let it dry fully before inspecting or moving to the next stage.
Finishing options after sanding
PETG does not accept all finishes the same way PLA does. A filler primer coat applied after sanding can smooth remaining micro-scratches and give paint something to bond to. Test any spray paint or clear coat on a spare piece first. Some solvent-based coatings react poorly with PETG. Water-based primers and paints are usually safer starting points.
Common mistakes with PETG
- Sanding too fast and creating a cloudy, smeared surface.
- Starting too coarse and leaving scratches that are hard to remove without more heat.
- Skipping wet sanding and wondering why the surface looks milky.
- Using solvent-based coatings without testing first.
- Applying too much pressure on curved geometry and losing the shape.
Bottom line
Sanding PETG is slower and more controlled than PLA, but the results are achievable. Start finer, move to wet sanding early, keep the surface cool, and test any finish coatings before committing. The material rewards patience and punishes speed.