Heat is the enemy, not the grit
PLA softens around 60C, and a power sander or an aggressive hand pass on dry paper gets there easily. The surface goes from matte to smeared and no finer grit will recover it.
Wet sanding carries the heat away and stops the paper loading with melted plastic. A bowl of water and a drop of dish soap is the whole technique.
The grit sequence
Each grit removes the scratches left by the one before it, so skipping steps means the final polish sits on top of visible 120-grit tracks. 120, 220, 400, 800 is enough for a part that will be painted. Continue to 1500 and beyond only if the part is meant to be glossy in its own right.
Change direction ninety degrees at each step. When you can no longer see scratches running the previous way, that grit is finished.
Materials that smooth chemically instead
ABS smooths in acetone vapour, which melts the outer surface into a gloss without touching the geometry underneath. PVB does the same in isopropyl alcohol, which is far less hazardous and needs no vapour setup.
PLA has no practical solvent, and PETG resists nearly everything convenient, so both are mechanical-only. That is a real reason to choose ABS or PVB for a part whose finish matters more than its convenience.
