What is wet-and-dry paper for, and how do I use it?

The lubricant is the whole point. Fine abrasive clogs almost instantly on paint, varnish and soft metal, and a clogged sheet burnishes rather than cuts. A film of water carries the swarf away continuously, so the paper keeps working, the scratch pattern stays even and the finish comes out flat rather than polished in patches.

What you'll need

• Wet-and-dry paper in the grits you need
• Water with a drop of washing-up liquid, or white spirit
• Rubber or cork sanding block
• Clean cloths
• Bucket or spray bottle

Step by step

1. Choose the lubricant for the surface.Water with a drop of washing-up liquid for fully cured paint, varnish, metal, plastic and body filler. White spirit for oil-based finishes where water might raise grain or find its way into a joint. Never water on bare timber, MDF or anything with an unsealed edge — it swells the fibres and the surface has to be sanded again once it dries.
2. Make sure the finish is genuinely cured.Wet sanding a finish that is touch dry but not hardened tears it and picks up gummy residue. Paint and varnish need days, not hours, before they will cut back cleanly. If the paper loads up with soft material, stop and wait.
3. Use a block.A rubber or cork block keeps the pressure even and the surface flat. Fingers behind the paper make small hollows and, on a curved surface, cut through the finish at the high points. Wrap the paper round the block and keep it taut.
4. Keep the surface flooded.Wet the surface, wet the paper, and keep adding lubricant as you go. If the slurry starts to feel sticky or the paper drags, add more liquid. A dry patch is where the scratches happen.
5. Use very light pressure.Let the grain do the work. Pressing hard drives the abrasive through the finish to the substrate — cutting through on an edge or an arris is the standard wet sanding mistake and it means recoating.
6. Work in one direction and check often.Straight strokes along the grain on timber, or in a consistent direction on a flat panel. Wipe the slurry off frequently and look at the surface: it should be uniformly dull with no shiny low spots left. Shiny patches mean the surface is not flat yet.
7. Work through the grits.Cutting back a run might start at 400 and finish at 800 or 1200. Polishing metal or plastic can go on to 2000 and beyond. Each grit removes the last one's scratches, so do not jump.
8. Clean up completely before recoating.Wash the slurry off, wipe with a clean damp cloth, and let it dry fully. Residue left on the surface stops the next coat adhering, and dried slurry in a corner shows through the finish.
9. Round off with polish where appropriate.On a cut-back gloss finish, a cutting compound and then a fine polish after the finest paper brings the shine back. On woodwork you are usually leaving the surface matt for the next coat instead.

Tips

• A drop of washing-up liquid in the water makes a noticeable difference — it stops the debris sticking to the paper and lets it flush away.
• Wet-and-dry works dry as well, and at fine grits on hard surfaces that is fine. You just lose most of the clog resistance that makes it worth buying.
• For cutting back a run in varnish, wrap the paper round a small hard block and work only on the run itself until it is level with the surrounding surface, then blend outwards.
• Keep used sheets — rinsed and dried, fine wet-and-dry lasts far longer than dry paper of the same grit.

Common mistakes

• Wet sanding bare timber or MDF — Water swells the fibres, raises the grain and can permanently blow an MDF edge. Wet-and-dry is for sealed and cured surfaces.
• Sanding a finish before it has cured — Soft finish tears and clogs the paper instantly, and the surface is left worse than before.
• Sanding with fingers instead of a block — Finger pressure cuts hollows and goes through the finish on edges and arrises.
• Letting the surface run dry — The moment the lubricant is gone the paper clogs and starts putting deep scratches into a finish you were trying to refine.

Questions people ask