Why Won’t My New Plaster Dry?

I’ve been plastering since 1983, and the question I get most from customers after any plastering job is: when can I paint it? The second is: why isn’t it drying?

Normal drying

Fresh plaster is full of water. A skim coat on a dry wall in a warm, ventilated room dries fairly quickly, and you’ll see it go from dark brown to an even light pink. Backing coats and renders take longer. In winter, in an unheated room or with the windows shut, it can take a lot longer. Uneven drying, with darker patches drying last, is normal at first.

When it’s not normal

• Patches that stay dark for weeks, or dry and then come back.
• Damp patches that show a pattern, such as low level, around a window, or under a chimney breast.
• White salts appearing on the surface.
• Plaster that feels soft or crumbles.

Common reasons

• The wall behind is still wet. If plaster went onto a wall that was damp from a leak, penetrating damp or rising damp, the moisture works its way out through the new plaster. A solid wall can hold a lot of water.
• Salts in the background. Hygroscopic salts from old rising damp or a chimney attract moisture from the air and keep the plaster damp. Gypsum plaster on a salt-contaminated wall is a common mistake.
• Condensation. A cold room with no heating or ventilation means the moisture from the plaster has nowhere to go and condenses back on the surface.
• The source hasn’t been fixed. New plaster on a wall still being fed by a leaking gutter or a bridged DPC will never dry properly.
• Paint too soon. Non-breathable paint or wallpaper over plaster that’s still drying traps the moisture.

What to do

• Keep the room warm and ventilated, and use a dehumidifier with the door shut if you need to speed things up, without forcing it with very high heat.
• Don’t decorate until it’s dry, and use a breathable mist coat first.
• If patches persist, have the wall checked for a moisture source or salts. Sometimes the plaster has to come off and be replaced with a salt-resistant system.