How do I sand between coats of paint or varnish?

Two things happen between coats. Dust settles into the wet finish and dries in as small hard nibs, and the surface dries glossy and slick. Denibbing deals with both: it takes the nibs off so the next coat is flat, and it gives the smooth surface enough of a key for the next coat to grip. It is not about removing the coat you just applied.

What you'll need

• 320–400 grit stearated paper, or a fine abrasive pad
• Sanding block for flat surfaces
• Tack cloth or lint-free cloth
• Good raking light

Step by step

1. Wait until the coat is hard, not just dry.Touch dry is not sandable. A finish that is still soft rolls up into gummy balls, tears, and clogs the paper instantly. Water-based paints usually need considerably longer than the tin suggests before they will cut back cleanly; oil-based ones longer still. If the paper gums up, stop and wait.
2. Use a fine grit and the right paper.320 to 400 grit for most work. Stearated silicon carbide paper — the pale, slightly chalky sort sold for finishes — resists clogging far better than ordinary aluminium oxide on paint. A fine non-woven abrasive pad is an excellent alternative on mouldings and awkward shapes.
3. Use almost no pressure.The weight of your hand and nothing more. You are removing dust specks and the tops of brush marks, not levelling the coat. Pressing hard cuts through to the previous coat or to the primer, especially on arrises and edges.
4. Use a block on flat work and fingers on shapes.A block keeps a panel or a flush door flat. On a moulding, a profile block or an abrasive pad follows the shape without rounding the detail off — corners and beads are where people sand through.
5. Check with a raking light.Hold a torch or a lamp almost parallel to the surface. Nibs, brush marks and missed areas jump out immediately in raking light and are invisible under normal lighting. This is how professionals check, and it takes seconds.
6. Aim for uniformly dull, not smooth-and-shiny.The whole surface should look evenly matt when you have finished. Shiny patches are areas the abrasive has not touched, and the next coat will not key there — that is where flaking starts later.
7. Deal with runs and heavy nibs separately.A run needs cutting back on its own with a small hard block, working only on the run until it is level with the surrounding surface, then blending. Do not try to sand a whole panel down to the level of a run.
8. Remove every trace of dust.Vacuum with a brush attachment, then wipe with a tack cloth or a barely damp lint-free cloth. Dust left on the surface is dust dried into the next coat, and the problem compounds coat by coat.
9. Do not sand the final coat.The last coat is left as it falls, unless you intend to cut back and polish it deliberately as a separate finishing operation.
10. Know when to skip it.Between coats of emulsion on a wall, denibbing is usually unnecessary unless the surface picked up debris. It matters most on woodwork, on gloss and satin finishes, and on varnish, where every nib shows.

Tips

• Sanding between coats matters more the glossier the finish. On a matt emulsion nobody will ever see a nib; on a gloss door every one catches the light.
• A folded piece of fine abrasive pad gets into a panel moulding far better than paper on a block and does not round the arrises.
• The first coat on bare timber raises the grain. Denibbing after that coat is the one that makes the biggest difference to the final feel of the surface.
• Change the paper often. Fine paper loaded with finish stops cutting and starts burnishing, which leaves shiny patches the next coat will not key to.

Common mistakes

• Sanding before the coat has hardened — The finish tears and rolls, the paper clogs instantly, and the surface ends up worse than before.
• Using too coarse a grit — Anything below about 240 leaves scratches that show through the next coat, particularly on gloss and on stained work.
• Pressing hard on edges and arrises — The coating is thinnest there and goes through to bare timber or primer, which then shows as a line once recoated.
• Recoating without removing the dust — Every particle left behind becomes a nib in the next coat, and the surface gets progressively worse rather than better.

Questions people ask