Why can I see my filled repairs through the paint?

Painters call it flashing. The colour is right and the coverage is right, but the repair reads as a dull or shiny patch in certain light. It is entirely about how much of the paint the surface absorbs.

What you'll need

• An acrylic primer or a stain block
• The topcoat, for a mist coat
• A roller and a brush
• Fine abrasive paper
• A dusting brush or damp cloth

Step by step

1. Confirm it is flashing, not coverage.Look at the wall from an angle in raking light. If the patch is the right colour but a different sheen, it is flashing. If it is a different colour, it is a coverage or a stain problem.
2. Understand the cause.Filler, new plaster and bare board are far more porous than aged paint. They pull the water and some of the binder out of the paint film, which changes how it dries and how it reflects light.
3. Sand the repair properly first.A ridge or a rough patch will show regardless of how well you prime. Sand flush and feather the edges well beyond the filler.
4. Dust off thoroughly.Filler dust on the wall makes the primer patchy in its own right. A damp cloth, then let it dry.
5. Prime the repairs.An acrylic primer, a dedicated plaster sealer, or a coat of the topcoat thinned about 10 percent with water. All three even out the absorbency.
6. Prime beyond the repair.Take the primer 50mm past the filler in every direction. A primer edge exactly on the filler edge simply moves the sheen change by a few millimetres.
7. Let the primer dry fully.Not touch-dry. Painting over damp primer produces exactly the effect you are trying to eliminate.
8. Consider priming the whole wall.Where a wall has many repairs, or is a mixture of old paint and new plaster, a full coat of primer over everything is faster and more reliable than spot-priming twenty patches.
9. Paint the whole wall, corner to corner.Never touch in over a repair. A patch of fresh topcoat on old paint always shows, because paint changes very slightly with age even in the same tin.
10. Keep a wet edge.Roll the wall in one session without letting the edge of the wet paint dry, so there are no lap marks to add to the problem.
11. Use a matt or flat finish where you can.Sheen differences are hardest to see in a flat matt and most obvious in a silk or an eggshell. Finish choice is a genuine lever.
12. Apply two full coats.One coat over a primed repair often still shows in strong side light. Two coats of topcoat over the whole wall is the reliable answer.

Tips

• Silk and mid-sheen paints show flashing far more than matt. On a patched wall in a room with a low window, matt is a much safer choice.
• A mist coat of the topcoat thinned 10 percent with water is a perfectly good primer for filler and often better than a general primer, because it is chemically identical to what goes on top.
• Check the wall with a torch held flat against it after priming. It shows every ridge and every absorbency difference before you commit to the topcoat.

Common mistakes

• Applying more coats of topcoat to hide it — Each coat is absorbed unevenly in the same way, so the sheen difference persists and often becomes more obvious as the film builds. The fix is to equalise the absorbency underneath, not to add more on top.
• Touching in over the repair only — A patch of new paint on an aged wall shows even when it came from the same tin, because paint yellows and dulls slightly with time and light. The whole wall has to be painted, corner to corner.

If it doesn't work

Questions people ask