Filling and Sanding Walls Before Painting

How to fill holes and cracks, sand smooth and spot-prime walls so paint looks even, with advice on filler types, drying times and common mistakes.

Paint magnifies surface faults. A wall that looks acceptable in daylight can show every hole, ridge and filler edge once a fresh coat goes on, especially in a sheen finish. Good filling and sanding takes time, but it is the single biggest influence on how the finished wall looks.

Check the walls first

Hold a torch at a shallow angle across the wall in a darkened or shaded room. This raking light shows dents, ridges, peeling paint and old filler.

Mark problem areas lightly with a pencil or masking tape. Look for:

• Nail holes and screw holes
• Hairline cracks and wider gaps
• Peeling or blown paint
• Raised edges where wallpaper or filler was previously removed
• Damp or mould, which should be dealt with first. See damp and mould before painting.

Choose the right filler

Ready-mixed fillers are convenient for small jobs; powder fillers are good value for larger repairs and often dry harder.

Steps for filling

1. Scrape loose material. Remove flaking paint and loose plaster with a scraper.
2. Undercut cracks. Widen hairline cracks slightly with the corner of a scraper so filler can grip.
3. Brush out dust. A dry brush or vacuum clears debris.
4. Apply filler. Press it in with a flexible filling knife, leaving it slightly proud of the surface because it may shrink.
5. Build deeper holes in layers. Let each layer dry before adding another, rather than filling in one go.
6. Wipe the edge. Draw the knife flat across to reduce sanding.

Steps for sanding

Allow filler to dry fully. Drying time depends on depth, temperature and humidity, and the manufacturer's instructions are the best guide.

1. Use a fine to medium-grit paper, often 120 to 180 grit, on a sanding block or pole.
2. Sand in light, circular or sweeping movements until flush.
3. Feather the edges so the repair blends with the wall.
4. Wipe dust with a barely damp cloth or vacuum.
5. Run your fingertips over the surface with your eyes closed. Faults are often easier to feel than see.

Wear a dust mask and eye protection. If the surface might contain old lead paint, stop and read lead paint in older homes before sanding.

Spot priming

Filled patches are more absorbent than surrounding paint and can show through as dull spots, called flashing. A spot coat of primer or a thinned first coat of paint seals them, so the final coats look even. On large areas of new plaster, a mist coat is better. See how to mist coat new plaster.

Common mistakes

• Filling before cleaning. Dust and grease prevent filler from sticking.
• Overfilling. Excess filler means more sanding.
• Sanding too soon. Damp filler clogs the paper and tears.
• Skipping spot-priming. The result is patchy.
• Ignoring cracks that keep returning. These may indicate movement or damp. Seek advice rather than repeatedly filling.

When to bring in a professional

Widespread cracking, bulging plaster, or walls with repeated damp need assessment. A decorator or plasterer can advise whether repair or reskimming is best. With Compito, you can include photos in your request and ask providers to say what preparation they would include.

A worked example: a bedroom wall with picture hooks and a crack

Take a bedroom wall with a dozen nail holes, a hairline crack running from the window corner and a dent from a bed headboard. Start by scraping any loose edges, then fill the nail holes with a light touch of filler on a flexible knife. Open the crack slightly so filler can bond, apply a flexible filler, and fill the dent in two thin layers. After drying, sand with fine paper on a block, wipe the dust, and spot-prime the repairs so the new paint does not look patchy. A small repair like this is often an hour or two of work plus drying time, and it determines whether the finished wall looks flat in side light.

Choosing sandpaper grit

Over-sanding with coarse grit can leave scratches that show through paint, so start only as coarse as the job needs.

Dust control

Sanding makes a lot of fine dust. Cover floors and furniture, open a window, wear a dust mask rated for fine dust and use a sander with extraction if you have one. Vacuum rather than sweep. If you are working on old paint in a pre-1980s home, treat it as potentially lead-containing and read lead paint in older homes before dry sanding.

Special cases

Rented homes. Small nail holes are often considered fair wear and tear, but check the tenancy agreement. Fill and touch up with the original colour if you have it.

Ceilings. Filling above your head is tiring and prone to uneven finish; use raking light to check and go gently.

Old plaster. Soft, powdery or hollow-sounding patches need more than filler. Tap lightly with a knuckle; a drum-like sound can mean the plaster has come away from the wall.

Between filling and painting

Wipe sanded areas, let the dust settle for a few minutes and take a last look under raking light. A repair that disappears under that light will usually disappear under paint. Anything still visible is worth another thin pass.

Frequently asked questions

Do I need to sand between coats?

Light sanding between coats on woodwork gives a smoother result. On walls, it is usually only needed where there are visible nibs or drips.

How long should filler dry before painting?

It depends on thickness and conditions. Small fills may be ready in a couple of hours, deeper repairs may take a day or more. Follow the product instructions.

Can I paint over cracks without filling?

Not successfully. Paint does not bridge cracks, and they will usually still show.

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