Why does my filler crack or sink after it dries?
Shrinkage, depth, movement and mixing — the four causes of a filled repair that reappears, and the technique that stops each one.
- Difficulty
- beginner
- Time
- 25 min
- Read
- 4 min
Short answer
Most filler shrinks as it dries, so a single deep fill sinks and a fill that is too wet cracks. Build deep repairs in layers, fill slightly proud, and use a flexible filler or caulk wherever the crack is in a joint that moves. If it keeps returning in the same place, the substrate is moving and no filler will hold it.
Filler failures fall into two families: the filler shrank, or the surface underneath moved. They look similar and need completely different fixes, so identifying which one you have is the whole job.
What you'll need
- appropriate filler for the depth and location
- filling knife
- abrasive and a sanding block
- primer
- flexible caulk (optional)
Step by step
- Look at the shape of the failure.A dished repair that has sunk below the surface is shrinkage. A clean crack straight through the middle of the repair is movement. They need different answers.
- Build deep fills in layers.Most fillers shrink in proportion to their thickness. Anything deeper than about 5mm should go in as two or more layers, letting each harden before the next.
- Fill slightly proud.Deliberately stand the filler a little above the surface, then sand back flush once it is hard. Filling level guarantees a slight hollow once it shrinks.
- Mix powder filler to the right consistency.Stiff, like thick toothpaste. Too much water increases shrinkage substantially and slows the set. Add powder to water rather than the other way round.
- Prepare the hole.Rake out loose material, undercut the edges slightly if you can so the filler is keyed rather than just sitting on top, brush out the dust, and dampen very porous plaster.
- Use a flexible product where things move.The junction between skirting and wall, or between two different materials, will always move. That needs decorators' caulk or a flexible filler, not a hard one.
- Use the right filler for the depth.Fine surface filler for skim repairs, a deep-repair or bonding filler for anything substantial, and an exterior or two-part filler outdoors.
- Let it harden fully before sanding.Sanding soft filler smears it and pulls it out of the hole. Harden means hard right through, which in a deep fill can be hours.
- Sand with a block.A fingertip dishes soft filler below the surrounding surface. A block keeps the repair flat with the wall.
- Prime the repair before painting.Filler is much more absorbent than the wall. Without a primer or a mist coat the topcoat sinks in and the repair shows as a dull patch.
- Find the movement if it keeps coming back.A crack that reopens in the same place every time is being driven by something — settlement, a joint between materials, a loose board. Filler is treating the symptom.
Tips
- Fill proud, sand flat. It is the single habit that makes repairs disappear.
- Two thin layers beat one thick one for every deep repair.
- If a crack has returned twice, stop filling it and work out what is moving.
Common mistakes
- One deep fill in a single application — Filler shrinks roughly in proportion to its thickness, so a deep fill sinks noticeably and often cracks as the outside sets before the middle.
- Using a hard filler on a moving joint — Rigid filler cannot accommodate the movement, so it cracks straight through — usually in the same place, within weeks. Caulk is the correct product there.
If it doesn't work
Repair visible as a shallow dip
Cause: Shrinkage, or sanded with a fingertip — Fix: Apply a second thin skim slightly proud and sand back with a block.
Crack straight through the middle of the repair
Cause: Movement in the substrate or the joint — Fix: Rake out, and use flexible caulk if it is a joint. If it is a wall crack, find the cause before refilling.
Filler crumbling out
Cause: Mixed too wet, dust left in the hole, or applied to a friable surface — Fix: Rake out completely, brush and dampen, and refill with a stiffer mix.
Repair shows as a dull patch through the paint
Cause: Unprimed absorbent filler — Fix: Spot-prime and repaint the whole wall or panel rather than touching in.
Questions people ask
Ready-mixed or powder filler?
Powder generally shrinks less, sets harder and is better for deep repairs. Ready-mixed is more convenient for small surface work but shrinks more and takes longer to harden in depth.
Can I fill a crack that keeps coming back with something stronger?
No. A stronger filler in a moving crack simply takes the plaster with it when it moves. Either make the joint flexible with caulk, or fix the movement.
How long before I can paint filler?
Until it is hard right through and dry, which is longer than touch-dry. A deep fill in cold weather can need a day. Painting damp filler traps moisture and the repair shows.