Leaving filler unprimed; it absorbs moisture and lifts the paint later Filling over paint or rust rather than clean bare metal Sanding filler dry without a mask — the dust is fine and irritant

Which sandpaper grit for car body filler?

80 grit to shape, 180 to refine, 320 before primer and 400-600 on the primer before paint.

Filler is easiest to shape in the short window after it has gelled but before it has fully hardened — the so-called cheese state, roughly twenty minutes in — and doing the bulk of the shaping then saves a great deal of effort. Every subsequent grit exists to remove the scratches of the previous one, and scratches that survive under primer will telegraph through the paint once the sun is on it. A guide coat makes the process visible: mist a contrasting colour over the area, and sanding removes it from the highs while leaving it in the lows. Filler must go over clean, abraded bare metal or a compatible primer, and it must never be left unprimed, because polyester filler absorbs moisture.

The options, ranked

Ranked best-first for the stated job. The third column is why the others exist.
Option	Good at	Bad at
80 grit	Shapes filler fast while it is still green, which is the easiest time to work it.	Leaves scratches deep enough to show through primer if they are not worked out.
180 grit	Removes the 80-grit scratches and refines the shape before priming.	Loads quickly if the filler is not fully cured.
320 grit	The right key for high-build primer to sit on.	Too fine to correct shape.
400-600 grit on primer	Flattens the primer so the basecoat has an even, non-absorbent surface.	Cutting through to filler means starting the primer stage again.
Guide coat	A light dusting of contrasting colour that reveals every low spot as you sand — the single most useful trick in the job.	An extra step people skip and then regret at the paint stage.

Technique that changes the outcome

Shape while the filler is still soft, then let it cure fully before refining
Use a long flat block on panels; a hand follows the dent instead of removing it
Apply a guide coat before each sanding stage
Feather the edges of the repair wide so there is no step under the paint

What goes wrong

Leaving filler unprimed; it absorbs moisture and lifts the paint later
Filling over paint or rust rather than clean bare metal
Sanding filler dry without a mask — the dust is fine and irritant

Compiled practical guidance rather than a published standard. Product formulations vary by brand and market, so read the label — and where the work is structural, electrical or gas, the regulations that apply where you are govern rather than anything here.

For	Use
Bare wood	80 to 120 to 150 on softwood, finishing at 180 or 220 on hardwood — and stop there, because over-sanding ruins stain.
Stripping paint	40 to 80 grit open-coat aluminium oxide or ceramic — but test for lead paint before sanding anything painted before the 1990s.
Rusty metal	60 to 80 grit to strip scale, 120 to 180 to blend — then prime the same day, because bare steel flash rusts within hours.
Between coats	220 to 320 grit with almost no pressure — you are knocking off dust nibs, not removing the coat.
Plasterboard filler	120 to 150 grit to level joint compound and 220 for the final pass, ideally on a mesh with a vacuum attached.
Plastic	Start at 320-600 wet for scratches, work up to 2000, then finish with a plastic polish — and keep it cool.
Final finish	400 to 600 dry, or 800 to 2000 wet on a fully cured film, followed by a polishing compound.
Wet sanding	Silicon carbide wet-or-dry paper, water with a drop of detergent, a rigid block, and 800 grit upwards.

Common questions

Which sandpaper grit for car body filler?: 80 grit to shape, 180 to refine, 320 before primer and 400-600 on the primer before paint.
What should I avoid?: Leaving filler unprimed; it absorbs moisture and lifts the paint later Filling over paint or rust rather than clean bare metal Sanding filler dry without a mask — the dust is fine and irritant

Which sandpaper grit for bare wood before finishing? 80 to 120 to 150 on softwood, finishing at 180 or 220 on hardwood — and stop there, because over-sanding ruins stain.
Which sandpaper grit removes paint? 40 to 80 grit open-coat aluminium oxide or ceramic — but test for lead paint before sanding anything painted before the 1990s.
Which sandpaper grit removes rust? 60 to 80 grit to strip scale, 120 to 180 to blend — then prime the same day, because bare steel flash rusts within hours.
Which sandpaper grit between coats of paint or varnish? 220 to 320 grit with almost no pressure — you are knocking off dust nibs, not removing the coat.
Which sandpaper grit for plasterboard and filler? 120 to 150 grit to level joint compound and 220 for the final pass, ideally on a mesh with a vacuum attached.
Which sandpaper grit for plastic? Start at 320-600 wet for scratches, work up to 2000, then finish with a plastic polish — and keep it cool.
Which sandpaper grit for a final finish? 400 to 600 dry, or 800 to 2000 wet on a fully cured film, followed by a polishing compound.
Which sandpaper should I use for wet sanding? Silicon carbide wet-or-dry paper, water with a drop of detergent, a rigid block, and 800 grit upwards.

Sources

Typical range compiled from manufacturer specifications — At One Place

How these figures are compiled and checked