How to Clean Plaster Walls. If your home or office interiors have plaster walls, consider cleaning them regularly as they tend to get dirty more often than painted walls. Plaster walls absorb moisture faster than painted walls and are more likely to attract dust and stains. The porous surface of plaster walls requires constant attention and cleaning.
Scrub the wall surface with a brush or damp cloth to ensure that the wall has an even finish. Plaster walls tend to be a little uneven on the surface, which makes them difficult to clean.
Use a strong solution of TSP (Tri-sodium Phosphate) mixed with water with a ratio of 1:15 (TSP: Water). You can also try using a non-phosphate substitute for water.
Use a dry non-abrasive scrubber, a soft brush or a sponge to rub the solution on the area of the wall which requires cleaning. The water will be absorbed by the wall surface but the Tri-sodium Phosphate action will disintegrate dirt and stains.
Allow the cleaned area to dry for few hours. Use a mild solution of TSP and scrub the surface again with a soft cloth to remove loose dirt off the wall. Do not scrape off or you will end up spoiling the wall surface.
Use an invisible (colorless) spray paint to fill out the porous surface of the wall to keep the wall surface clean.