Letting the surface run dry mid-stroke Working on a thin coat or on an edge, where you will cut through in seconds Skipping grits — the coarse scratches simply survive to the polishing stage

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.

Wet sanding is not about the water lubricating the cut; it is about removing swarf and heat. Loose particles that stay on the surface are exactly what put deeper scratches into it, and heat is what softens paint and plastic into a smear. So the technique is: plenty of water, frequent rinsing of both the paper and the surface, and consistently light pressure. Use a rigid block wherever the goal is flatness, since fingers follow every ripple, and a soft interface pad only where the shape is curved. Check progress constantly by squeegeeing the surface dry — a wet surface hides exactly the defects you are trying to remove.

The options, ranked

Ranked best-first for the stated job. The third column is why the others exist.
Option	Good at	Bad at
Silicon carbide wet-or-dry paper	Waterproof backing and a sharp friable grain that keeps cutting; the standard for paint, clearcoat and plastic.	Breaks down faster than aluminium oxide when used dry.
Aluminium oxide paper	Tougher and longer-lasting for dry work on wood and metal.	Its backing is usually not waterproof, and it clogs on wet paint.
Foam-backed abrasive pad	Follows curves without cutting through on the high points.	Cannot flatten a surface, because it follows what is already there.
Water with a drop of detergent	Cuts surface tension so the water spreads and floats swarf out of the abrasive.	Must be rinsed off completely, or the residue interferes with polish.

Technique that changes the outcome

Soak the paper for a few minutes before starting
Add a drop of detergent to the water so it spreads instead of beading
Rinse the paper every few strokes and keep the surface flooded
Use a block for flat panels and change direction between grits so you can see the previous scratches disappear
Squeegee dry to inspect before moving up a grit

What goes wrong

Letting the surface run dry mid-stroke
Working on a thin coat or on an edge, where you will cut through in seconds
Skipping grits — the coarse scratches simply survive to the polishing stage

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.
Car body filler	80 grit to shape, 180 to refine, 320 before primer and 400-600 on the primer before paint.
Final finish	400 to 600 dry, or 800 to 2000 wet on a fully cured film, followed by a polishing compound.

Common questions

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.
What should I avoid?: Letting the surface run dry mid-stroke Working on a thin coat or on an edge, where you will cut through in seconds Skipping grits — the coarse scratches simply survive to the polishing stage

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

Sources

Typical range compiled from manufacturer specifications — At One Place

How these figures are compiled and checked