Why are there lighter patches on a freshly plastered wall?
Lighter patches on freshly plastered walls are usually caused by uneven porosity, residual moisture, surface salts or alkaline conditions in the plaster.
Fresh plaster can dry unevenly and may have different levels of porosity across the wall. These areas can absorb paint at different rates, causing lighter or patchy colour. Residual moisture, efflorescence or alkaline minerals in the plaster can also affect the appearance of the coating.
Allow the plaster to dry and stabilise properly, correct any moisture source, remove loose salts or powdery material, and prepare the surface thoroughly before repainting. On new, friable or highly porous plaster, use a suitable ACRO plaster primer or bonding/sealing system to stabilise the surface and provide a more uniform base before applying the decorative topcoat.