Do I need to sand between coats of paint?

Two entirely different reasons get called sanding between coats, and confusing them is why people either skip it when it matters or waste an evening doing it when it does not. One reason is smoothness: shearing off nibs, dust, brush marks and raised grain so the next coat starts flat. The other is adhesion: cutting a key into a coat that has cured too hard for the next one to bite into chemically.

Step by step

1. Work out which of the two jobs you are doing.Smoothness matters on anything you will see in reflected light — doors, skirting, window frames, furniture, any gloss or satin surface. Adhesion matters whenever a coat has been left long enough to cure hard, or when the next coat is a different type of paint from the one below.
2. Sand trim between every coat.Primer, undercoat and between topcoats. A light pass with a fine abrasive or an abrasive pad, just enough to take the shine off and remove any nibs, then dust off completely with a vacuum and a tack cloth or a barely damp cloth. This is most of what separates a good painted door from a poor one.
3. Use a fine grade and almost no pressure.Something in the 240 to 400 range on a finish coat. You are levelling the top of the film, not cutting it back. Too coarse leaves scratches that the next coat shows; too much pressure cuts through to the layer below and gives you a pale patch.
4. Leave walls alone unless something specific is wrong.Emulsion coats bond to each other perfectly well within the recoat window and the finish is matt enough that nibs rarely show. Knock off a specific run, a bit of grit or a raised patch with a fine abrasive on a block, and get on with the second coat.
5. Always sand if you have missed the recoat window.Modern water-based trim paints in particular bond chemically to a coat that is still curing. Once fully cured they are hard and closed, and the next coat sits on top of them. If it has been days rather than hours, abrade the whole surface before recoating — this is the single commonest cause of trim paint peeling in sheets.
6. Sand before a change of paint type.Water-based over solvent-based, any topcoat over a hard-cured undercoat, or a different brand's system: abrade first. A key is cheap insurance where the chemistry may not agree.
7. Dust off properly, or you have made things worse.Sanding dust that stays on the surface is picked up by the next coat and becomes the nibs you were sanding off. Vacuum, then wipe, then let it dry before you paint.

Tips

• An abrasive pad follows mouldings, beading and turned detail where paper cannot go, and is the practical tool for skirting and architrave.
• On a large door, sanding between coats with a sanding block keeps the surface flat; sanding freehand rounds the edges over.
• If the surface is already smooth and you are inside the recoat window on a wall, just paint. Sanding for the sake of it costs an evening and gains nothing.

Common mistakes

• Sanding hard enough to cut through to the undercoat — You expose a layer of a different colour and absorbency, which then shows through the topcoat as a patch. Very light passes, fine grade.
• Recoating trim days later without abrading — Modern acrylic trim paints cure hard and closed. The next coat has nothing to bond to and eventually peels off in a sheet.
• Sanding and then painting without removing the dust — The dust ends up suspended in the wet film. You have created exactly the defect the sanding was meant to remove.

Questions people ask