Powder filler or ready-mixed — which should I use?

The two products behave differently for one reason that explains almost every practical difference between them. Ready-mixed filler is solid material suspended in water: as the water leaves, the volume it occupied disappears, and the filler sinks. Powder filler is mixed and then sets chemically, like plaster, so the water becomes part of the set material rather than evaporating out of it.

Step by step

1. Measure the depth of what you are filling.A screw hole or a hairline is shallow and ready-mixed is fine. A hole several millimetres deep, a chased channel or a missing corner is where shrinkage becomes visible, and that is powder filler territory.
2. Use ready-mixed for convenience where depth is small.No mixing, no waste, no working time pressure, a smooth consistency straight from the tub, and it sands very easily. For filling nail holes and small dents before painting, it is the sensible choice.
3. Use powder filler where shrinkage would show.It sets rather than dries, so it holds its volume. It can be built up in a much thicker section in one application, it is harder when set, and it can be mixed to whatever consistency the job needs.
4. Respect the working time of a setting filler.Once mixed you have a fixed window — often twenty to sixty minutes depending on the grade — and it goes off in the bucket whether or not you have finished. Mix small batches, and never add more water to soften a batch that has started to set, which destroys the strength.
5. Fill deep repairs in coats regardless of the product.Even a setting filler is easier to control in two or three coats, and a ready-mixed one has to be. Each coat thin enough to dry or set through, each allowed to harden before the next.
6. Leave the final coat proud and sand back.This is the discipline that survives shrinkage. A repair filled dead flush will always end up slightly hollow with a ready-mixed product. Fill proud, let it harden fully, sand flush.
7. Check whether the wall is sucking the water out.A very dry, dusty or freshly plastered surface pulls water out of the filler before it can set or dry properly, which leaves it powdery. Damp the surface or seal it with a diluted primer first.
8. Match the grade to the job rather than buying one tub for everything.Fine surface filler for hairlines and finishing, general filler for ordinary holes, deep-fill or lightweight filler for large voids, exterior filler for outside, two-part chemical filler where the repair will be drilled or loaded. They are not interchangeable and each is cheap.

Tips

• Powder filler is far cheaper per volume and it keeps indefinitely if stored dry. Ready-mixed is a convenience product and it is finished once the tub has stiffened.
• Mix powder filler by adding powder to water, not water to powder, and let it slake for a moment before mixing. It goes lump-free far more easily that way.
• Never mix filler in the tub or bucket you mixed the last batch in without cleaning it. Set filler on the sides accelerates the new batch dramatically.
• A repair in a wall that moves — a crack that keeps reopening — is not a filler problem. Filler is rigid, and a moving crack needs a flexible product or the movement dealt with.
• Ready-mixed filler frozen in a cold garage is ruined permanently. It goes grainy and will not recover.

Common mistakes

• Filling a deep hole in one pass with ready-mixed — The face dries first and the wet middle pulls it in as it dries, so the repair sinks and cracks round the edge. Build up in coats or use a setting filler.
• Adding water to revive a setting filler that has started to go off — It breaks the crystal structure that is forming. The result is weak, powdery and will not sand or take paint properly.
• Filling dead flush — Almost every filler shrinks a little. Fill proud and sand back; it takes the same time and the repair does not reappear.
• Using one all-purpose filler for every job — Fine filler will not take depth, lightweight filler will not take a fixing, and interior filler will not survive outdoors. Each costs very little.

Alternatives

• Lightweight or deep-fill filler: A foam-bodied ready-mixed product that can be applied very thick in one pass without slumping and sands extremely easily. Softer than a setting filler, so not for anything that takes wear or a fixing.
• Setting powder filler: Mixed from dry and cured chemically. Minimal shrinkage, deep fills in one go, hard when set. Needs mixing and has a working time.
• Fine surface filler: Very smooth ready-mixed material for hairlines, small imperfections and the finishing coat over a deeper repair. Not for depth.
• Two-part chemical filler: Resin and hardener, extremely hard and dimensionally stable, sets in minutes. For repairs that will be drilled, loaded or exposed outdoors.

Questions people ask