Sanding is the unglamorous step that makes everything after it look good. On an FDM part, careful sanding knocks back layer lines and blemishes so primer and paint sit smoothly. Rushed sanding, on the other hand, leaves swirls and flat spots that show through the finish.
Work through the grits
The reliable approach is to start with a coarser grit to remove the worst of the layer lines, then step up through progressively finer grits to refine the surface. Skipping grades leaves scratches that the next stage cannot hide.
Wet sanding the finer stages keeps dust down and gives a smoother result, particularly before painting.
Knowing when to stop
Over-sanding can soften crisp edges and erase detail, so the goal is a smooth surface, not a reshaped part. We sand to suit the finish the part needs, whether that is a quick tidy-up or a glass-smooth base for airbrushing.