Flashing and patch marks showing through the finish
9 June 2026 · 3 min read
The wall looks fine until the afternoon light hits it, and every repair reappears. Why it happens and how it is actually fixed.
You paint a wall, it looks perfect, and then late-afternoon light rakes across it and every patch, every joint and every repair is visible as a slightly different sheen. Nothing has failed. The colour is uniform. It just looks wrong.
This is flashing, and it is a preparation problem rather than a paint problem.
What causes it
Paint sheen is produced by how smooth and how sealed the dried film is. A film sitting on an absorbent surface loses some of its binder into that surface, ends up thinner and rougher at the microscopic level, and reflects light differently from the same paint over a sealed surface.
So wherever the wall's porosity changes, the sheen changes:
- **Filler and patching compound** are far more absorbent than aged paint
- **Plasterboard joint compound** is more absorbent than the paper facing
- **Bare plasterboard paper** is more absorbent than a painted surface
- **Sanded-back areas** are more absorbent than untouched paint
- **New render** is more absorbent than old
The colour is identical everywhere. Only the reflectance differs, which is why it disappears in flat light and reappears when the sun is low.
Why it is worse now than it used to be
Two reasons. Low-sheen and matt finishes are fashionable, and the flatter the finish the more it exaggerates porosity differences. And modern LED downlights mounted close to walls throw hard raking light, which is close to a laboratory test for surface uniformity.
Where it always shows
Long unbroken walls, hallways with a light at one end, walls beneath a window, and anywhere a downlight is within about 300 mm of the surface. If your wall has any of these, the preparation standard has to be higher.
The actual fix
**Spot prime every repair.** Non-negotiable. Any filled, sanded or bare area gets primer before the finish coats. This equalises porosity so the topcoat behaves the same everywhere.
**Seal whole walls where the substrate has changed.** If more than roughly a fifth of a wall has been repaired, prime the entire wall rather than the patches. A patchwork of primed and unprimed areas has just moved the problem.
**Use a full-bodied sealer on new board.** Not a thinned topcoat. Plasterboard sealers are formulated to sit on the paper and equalise it with the joints.
**Do not over-sand the paper.** Sanding through the face of plasterboard raises the paper fibres and creates a permanently more absorbent area. That patch will flash through almost anything.
**Coat wall to wall, wet edge maintained.** Stopping and restarting mid-wall leaves a lap that reads as a sheen difference in raking light.
Fixing it after the fact
Once it has happened, touching up the affected areas makes it worse — you are adding another layer of differential build.
The fix is: prime the whole affected wall, then apply two full finish coats wall to wall. It is annoying, it is the only reliable answer, and it is why the spot-priming step is worth insisting on beforehand.
Choosing a finish that is kinder
If your walls are old, much-repaired, or lit by raking downlights, a low sheen will hide considerably more than a matt, and a matt will hide more than an eggshell. There is a genuine trade-off — higher sheens wash better — but on a difficult wall the finish choice does more than the preparation can.