Sanding a wall before painting: choosing the right grit

Sanding a wall before painting is the step most often skipped, yet it is what determines the hold and appearance of your paint. A correctly sanded wall offers the paint a slightly rough, regular surface on which it grips; an unsanded wall, glossy or dusty, makes the paint flake and reveals every defect. The choice of sandpaper grit is the first decision to make. Here is how to sand a wall correctly, without mistakes, with the right grit at each step.

Why sand before painting: the three reasons

• Create adhesion: a glossy or too-smooth surface repels the paint, which then peels
• Flatten the defects: filler joints, spatula marks, old runs and flakes are softened by sanding
• Even out the absorption: a regular surface dries homogeneously, with no matt or glossy areas

A distinction is made between preparation sanding, which removes the bulk and makes the defects disappear, and finish sanding, very light, which will dull a coat of paint or a filler for the next coat. Do not confuse them: sanding too hard on a finish damages the surface, sanding too gently on a filler is useless.

Which grit to choose depending on the support

• Grit 60 to 80: removal of flaked old paints and significant roughness
• Grit 120: roughing of filler joints and small irregularities, the standard choice for a plaster wall
• Grit 180 to 220: finish sanding between two coats of paint or on a smoothed filler
• Grit 240 and above: light dulling of an alkyd paint before a new coat

Dry or wet: making the right choice

Dry sanding (ordinary sandpaper) suits filler and plaster: quick, it produces dust that will need to be removed. Wet sanding (wet-and-dry paper, grit 240 or more) is reserved for finishes on smoothed filler or on glossy paint: it avoids dust and gives a very smooth surface, but requires letting it dry completely before painting. For a standard wall to repaint, dry sanding at grit 120 is enough in the vast majority of cases.

The gear for sanding a wall

• Orbital or belt sander for large flat surfaces (with dust extraction if possible)
• Sanding block or multi-tool sander for the corners and small areas
• Sandpaper sheets of the chosen grits, cut to the size of the block
• FFP2 protective mask: filler and paint dust is very fine and irritating
• Vacuum cleaner with soft brush and microfibre cloth for the final dust removal
• Abrasive sponge for wet finishes

The method: sanding crossways without pressing

1. Protect the floor and furniture, mask the skirting boards and door frames
2. Start with the damaged areas (flakes, joints) at grit 80 or 120 in circular movements
3. Sand the large surfaces at grit 120 in crossed passes (vertical then horizontal) to avoid hollows
4. Do not force: let the weight of the tool do the work, excessive pressure digs into the plaster
5. Check the flatness with the flat of your hand and the evenness under grazing light
6. Finish with a complete dust removal with the vacuum cleaner then a damp cloth, and let it dry

Dust removal, the step everyone forgets

A sanded wall covered in dust will give a pitted, poorly adhering paint. Vacuum the whole surface with a soft brush, then a slightly damp microfibre cloth. Let it dry 30 minutes before applying the primer or paint. If you sand between two coats of paint, remove the dust the same way: it is what avoids the little bubbles and grains under the finish.

The mistakes to avoid

• Sanding without a mask or extraction: filler dust is very fine and irritating
• Pressing hard on the sander, which digs visible hollows under the light
• Sanding in one direction only, which leaves streaks, especially on filler
• Using too coarse a grit on a finished filler: scratches appear under the paint
• Painting immediately after sanding without dusting or letting it dry

Particular cases: glossy paint, wallpaper, damp walls

On an old glossy or satin paint, a simple dulling sand at grit 180 to 220 is enough before the new coat; on wallpaper to be painted, sand very lightly at grit 120 and check the hold of the strips. On a damp wall or with mould stains, above all do not paint before treating the humidity problem at the source: sanding solves nothing and the mould will reappear under the paint. In that case, treat with bleach water, let it dry then sand and apply a suitable primer.

With the right grit and a rigorous method — sanding crossways without pressing, dusting and letting dry — you prepare a wall that will receive its paint in the best conditions. Allow 30 to 45 minutes to sand and dust a 15 m² room, and you will see the difference from the first coat: a uniform surface, free of pinholes and marks, that grips perfectly and lasts over time.