How do I fill a deep hole in a wall so it does not sink?

A hole deeper than a centimetre or so behaves differently from a nail hole. Filler shrinks as it dries, and the amount it shrinks is proportional to how much of it there is, so a single deep application always pulls away from the edges and dishes in the middle. It also often stays soft in the centre for a long time. Layering solves both.

Step by step

1. Establish what is behind the hole first.A hole right through plasterboard into a cavity cannot be filled at all — filler has nothing to sit on and will simply fall through. That needs a patch, which GuideHQ covers separately. This guide is for holes with solid material behind them.
2. Rake back, undercut and clean the hole.As with any filling: firm edges, slightly undercut so the repair is trapped, and free of dust.
3. Choose a filler suited to the depth.Deep-fill, bonding and repair products are made to be applied thickly without slumping or excessive shrinkage. Fine surface filler is not, and using it for depth is the usual mistake.
4. Fill the first layer hard against the back and sides.Press it in firmly to eliminate voids. Do not try to reach the surface with this layer — aim to fill roughly the back half of the hole.
5. Score the surface of each layer before it sets.A few scratches with the corner of the knife give the next layer a key. It takes seconds and makes the repair a single mass rather than a stack of discs.
6. Let each layer set properly before adding the next.Adding to a soft layer traps moisture and simply recreates a single deep mass. Wait for it to firm up.
7. Finish with fine surface filler, slightly proud.The last few millimetres want a fine filler that sands cleanly. Leave it standing marginally above the wall so you sand down to flat, rather than level so you sand into a dip.
8. Sand flat, then seal before painting.Filler is more porous than the wall around it. Painted straight over, it dries to a different sheen and shows. Seal or spot-prime the repair.

Tips

• For a deep hole in a corner or an edge, a temporary board taped across one face gives the filler something to work against.
• Check the repair with a straightedge or the edge of the filling knife rather than by eye. A shallow dip is invisible until the wall is painted and lit.

Common mistakes

• Filling a deep hole in one application to save time. — It shrinks, dishes, cracks and often stays soft in the middle. The second attempt takes longer than layering would have.
• Finishing flush with the wall. — Filler shrinks slightly as it dries, so flush becomes slightly hollow. Proud and sanded back is the reliable method.

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