An acrylic coating dries by losing water to the air. When the air is already full of water, it dries slowly — and a film that has not formed properly when the next shower arrives will wash, streak or fail to bond. None of this means you cannot paint in the rains. It means you have to work with them.
The wall has to be dry, not just the sky
A blockwork wall holds water for days after heavy rain, and the surface can feel dry while the substrate behind it is still saturated. Paint applied over it traps that moisture, and the film blisters as the sun drives it back out.
Give an exterior wall at least two clear days after real rain before you coat it. A cheap moisture meter pays for itself on the first job.
Watch the window, not the forecast
What matters is the hours immediately after application. A weather-shield acrylic wants a few hours of dry weather to form its film properly.
In practice that means starting early. Paint from first light through mid-morning, stop before the afternoon build-up, and leave the elevation that faces the incoming weather until you have a settled run of days.
Humidity slows everything down
Recoat times printed on a tin assume moderate conditions. In high humidity, double them and check by touch rather than by clock.
The test is simple: press a thumb firmly into an inconspicuous area. If it leaves a print, it is not ready for the next coat, whatever the tin says.
Harmattan is the opposite problem
In the dry season the risk reverses. Paint skins over fast, dust settles into a wet film, and a roller can drag before it has levelled.
Work in smaller sections, keep a wet edge, and avoid painting a wall in full afternoon sun — the film sets before it has flowed out, which is what leaves roller marks and lap lines.
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