The simple rule
Use filler for holes and cracks in flat surfaces that need to be sanded smooth. Use decorator's caulk for gaps along joints between two surfaces that move slightly – where skirting meets the wall, round door frames, along the top of worktops and between wall and ceiling.
Filler sets hard and can be sanded flat. Caulk stays flexible and can't be sanded, but it moves with the building, so the gap doesn't open up again.
| Job | Use |
|---|---|
| Gap between skirting and wall | Caulk |
| Gap round door and window frames | Caulk |
| Crack where wall meets ceiling | Caulk (or flexible filler if wide) |
| Nail and screw holes | Filler |
| Dents and gouges in plaster | Filler |
| Hairline cracks across a wall | Fine surface filler |
| Wide cracks in plaster | Filler, possibly with scrim tape |
| Holes in woodwork | Wood filler |
| Round baths, showers and sinks | Silicone sealant (not caulk) |
Caulk or filler?
Decorator's caulk explained
Decorator's caulk is an acrylic sealant that comes in a cartridge and is applied with a skeleton gun. It stays slightly flexible after it dries, can be painted over with water-based paint, and gives crisp, clean lines at joints. Most decorators run a bead of caulk along every skirting board, architrave and frame before painting – it's one of the things that makes a professional job look finished.
Caulk has limits. It can't be sanded. It shrinks a little as it dries, so very wide gaps need two applications or a backing rod first. It isn't waterproof enough for wet areas – around baths and showers, silicone is needed, and silicone can't be painted.
Fillers explained
Filler should be applied slightly proud of the surface, left to dry fully and then sanded flush. Deep holes are filled in layers. Painting over damp filler is a common cause of sunken patches and cracking.
- Ready-mixed fillers – convenient for small holes and dents
- Powder fillers – mixed with water, stronger and better for larger repairs
- Lightweight fillers – don't shrink or slump, good for deeper holes
- Fine surface fillers – for hairline cracks and final skims
- Flexible fillers – for cracks that keep reappearing
- Two-part wood fillers – very hard, for exterior and damaged woodwork
Rather leave it to a professional?
Common mistakes
- Using caulk on flat wall surfaces – it can't be sanded and always shows
- Using filler in a moving joint – the crack comes straight back
- Painting caulk too soon – it can crack the paint film
- Using caulk round a shower instead of silicone
- Leaving filler patches unprimed – they can 'flash' through the finish coat
Cracks that keep coming back
If a crack reopens after every repaint, the building is moving there – often at the junction of walls and ceilings, around door frames or along plasterboard joints. A flexible filler or caulk, or scrim tape and a skim of filler over the joint, can help. Large, widening or diagonal cracks may indicate a structural problem and should be checked before decorating.
For more on preparing surfaces, see how to prepare walls before a painter and why paint peels and cracks.
Leave it to the decorator?
Good preparation is a large part of any decorator's price, and filling and caulking are a big part of that. When comparing quotes, ask whether filling, sanding and caulking are included. Get quotes from local painters and decorators.
