What grit for sanding body filler?

Polyester body filler is hard and stands proud. With P80 on the random orbital sander or the hand sanding block it takes shape quickly; starting finer only costs time and discs. On the hand block you sand flat surfaces – the block shows you where material is still missing.

With P120 and P180 you feather the edges into the old paint until you cannot feel a step. P240 is the last step before primer surfacer: the primer fills the scratches of P240, not those of P80.

Finishing filler on small spots: P180 to P240, usually by hand. Then primer surfacer or primer, and you carry on with P320 to P400 – see “What grit before painting?”.

Primer surfacer needs a key: the scratches of P180 to P240 give it grip. A filler repair sanded almost smooth with P400 offers less of that, and the fine disc quickly clogs on the soft filler. Fine sanding only starts on the primer surfacer.

Apply guide coat to the filler, then go over it with the hand block: whatever stays black sits lower and needs more filler. Also run your flat hand over the area – ideally wearing a glove; you often feel steps before you see them.

The most common mistake is sanding past the filler into the old paint with P80. Primer surfacer does not reliably fill coarse scratches in the paint. So feather the edge with P120 to P180 first.

Filler applied too thick costs the most time when sanding. Better to apply several thin layers and rough-sand in between.