The Frustration of the Patchy Wall#
You buy quality paint, prepare the room, and watch your fundi apply two coats. While the wall is wet, it looks rich and uniform. But hours later, as the water evaporates and the afternoon light hits the room from the balcony door, patches appear.
Some spots look dull, other areas look shiny, and faint vertical lines show where the roller overlapped. In the trade, this defect is known as "flashing" or "banding."
Before rushing to blame "bad paint," it helps to understand the physics of film formation. Paint does not dry like water evaporating from a concrete floor; it cures as a polymer film, and how it cures depends entirely on what lies beneath it.
Cause 1: Differential Suction from Unprimed Filler#
When you repair cracks with polyfilla or plaster of Paris, that filler is significantly more porous than the surrounding cured cement plaster.
If you roll paint directly over the filled wall without priming, the dry filler acts like a dry sponge. It instantly sucks the water and resin binder out of the wet paint before the film can coalesce properly.
The result is a dull, under-bound spot on your wall that looks lighter and chalkier than the rest of the surface. To prevent this, professional painters always "spot-prime" all filled areas with plaster primer or a thinned mist coat before applying the main coats.
Cause 2: Breaking the Wet Edge and Rolling Backwards#
Paint binders start skinning over within 10 to 15 minutes of application, especially in Kenya’s warm climate.
If a painter rolls a section, takes a tea break, and then resumes painting where they left off, the new wet paint overlaps partially dried paint. The roller picks up and tears the delicate drying skin, creating a raised double-thickness line that catches the light.
Always work systematically across a wall from corner to corner. Roll in a "W" or "M" pattern to distribute the paint, then lay off with gentle, continuous downward strokes in one consistent direction while the edge remains wet.
Cause 3: Over-Diluting Paint with Water#
It is common practice for fundis to add water to interior emulsions to make rolling effortless. While manufacturer technical data sheets permit 5% to 10% water dilution for the first coat on dry masonry, excessive dilution ruins the formulation.
When paint is thinned by 25% or 30%, the resin cannot hold the pigment particles in suspension. The paint runs unevenly, drips off rollers, and dries with streaky, see-through patches.
Step-by-Step Guide to Fixing a Patchy Wall#
- Step 1: Allow the patchy wall to dry completely for at least 24 hours so you can inspect the true defect under full daylight.
- Step 2: Lightly sand down any raised lap marks or roller ridges using fine 180-grit or 220-grit sandpaper.
- Step 3: Spot-prime any porous filler spots or bare plaster patches with an alkali-resistant plaster primer.
- Step 4: Stir your paint thoroughly from the bottom of the tin with a clean paddle for two full minutes to remix settled pigments.
- Step 5: Apply one continuous, full-bodied coat across the entire wall without stopping midway, maintaining a wet edge from start to finish.