Decorating following a damp proofing treatment:
When decorating new plaster following a damp proofing treatment you have to allow the plaster longer to dry than when you have had just a skim plaster coat.
When will the plaster be dry enough to paint or wallpaper?
Following a remedial damp proofing treatment you should wait 6 months before painting: with a mist coat first see below; but only with a water based emulsion and 12 months if you are wallpapering or painting with a silk emulsion.
Why does the plaster take longer to dry?
Plaster takes much longer to dry following a damp proofing treatment because of the water proofing agents used within the sand and cement undercoat plaster.
Under normal conditions the majority of water within a render mix is sucked up by the mortar and brickwork of the wall, very little water evaporates into the room. As a consequence, if no waterproofing agents are used the plaster will be dry enough to paint, within 2 to 3 weeks if a sand and cement undercoat render is used and a few days if just a skim coat is applied.
However, the use of a water proofing agent within the render mix prevents the wall sucking the water out of the mix, consequentally the water has to slowly evaperate into the room.
What happens if you paint too soon?
If you paint or wallpaper before the moisture has evaporated, the moisture may blister the paint or blow the wallpaper in areas, while it is evaporating. This is the chance you take if you paint before the reccomended drying out times above.
A mist or ghost coat.
When painting new plaster you must use a mist or ghost coat; see opposite; which is a 50 - 50 mix of water based emulsion and water.
Paint (not primers) sit on top of a surface, by diluting the paint with water the paint molecules are sucked into the new plaster providing a key for further coats of paint. Without a mist or ghost (primer) coat there is a risk of the paint blistering.
Points to remember:
- Always use water based paints.
- Dilute the paint with 50% water.
- Apply 2 mist coats.
- Do not use oil based paints.
- Do not use vinyl silk paint.
- Do not use PVA as a primer.