How long should filler dry before I paint it?

There are three separate questions hiding in this one. Is it dry enough to sand, which is about surface hardness. Is it dry enough to paint, which is about the moisture deeper down. And will it show through the paint, which is about absorption rather than drying at all — and which is why a perfectly dry repair can still be visible after two coats.

Step by step

1. Judge by hardness, not by the clock.Press a fingernail into the repair at its deepest point. It should feel uniformly hard with no give and no cool damp feel. The stated drying times on a tub assume a shallow application at twenty degrees in a ventilated room, which is rarely what you have.
2. Expect depth to dominate the timing.Drying time rises sharply with thickness because the water has to travel out through the material. A repair three times as deep takes far more than three times as long. This is the main reason for building up in coats.
3. Account for the conditions.Cold, humid, still air slows drying dramatically. A newly plastered room in winter with the windows shut is the worst case. Ventilation helps more than heat, because heat without airflow just holds moisture in the room.
4. Remember that setting filler is different.A setting powder filler and a two-part filler cure chemically rather than by drying, so they are hard and paintable much sooner and their timing barely depends on conditions. It is ready-mixed filler that keeps people waiting.
5. Sand once it is hard, then dust off thoroughly.Vacuum and then wipe with a slightly damp cloth. Filler dust left on the wall stops paint keying and shows as a dusty bloom through the first coat.
6. Prime the filled area before painting.This is the step that stops the repair showing. Filler is more absorbent than the surrounding painted wall, so it drinks the first coat and dries duller — the effect decorators call flashing. A coat of primer or a thinned mist coat over the repair evens the absorption out.
7. Prime the whole wall if there are many repairs.Spot-priming a dozen patches leaves a dozen slightly different absorbencies. On a heavily filled wall, priming the lot is quicker than chasing patches through three topcoats.
8. Do not paint over a repair that is still cool to the touch.Trapped moisture under a paint film causes blistering, poor adhesion and, on an exterior repair, frost damage the first cold night.

Tips

• If you need the wall painted today, use a setting powder filler or a two-part filler rather than trying to rush a ready-mixed one.
• A dehumidifier in a closed room dries repairs faster than a heater, because it removes the moisture rather than just warming the air.
• Filler that has dried at the surface but not underneath often looks slightly darker in the middle of the repair. Raking light shows it clearly.
• On exterior work, never paint a repair that could still hold water before a frost. The freeze will lift the paint and the filler with it.
• Two thin coats of finish over a primed repair hide it better than one thick coat, and the difference is most obvious in raking light.

Questions people ask