Plastering over thermalite blocks

by Paul
(Preston)

Hi Paul,

I bought your DVD a couple of years ago and have since had great success skimming walls and ceilings.

I've got a wall with thermalite blocks with a new concrete lintel amounst them. What is the best way to plaster them, when I built the wall the suction was immense, do I need to pva before using hardwall or can I just wet them down.

What about the lintel, I obviously don't want to buy a different plaster just to do a lintel but it's low suction so can I use the hardwall over it?

thanks for your help

Regards
Paul

Answer

Hi Paul,
As you say the suction will be very high so either apply PVA or damp down with water. Pros and cons?
Damping down with water means that you can see when you have controlled the suction as the water just runs down the wall - but can take a bit longer to dry out for decoration (only normally a few days extra).
Using PVA means you are not introducing loads of water that needs to dry prior to decoration but you are not able to gauge how well you are controlling the suction as you are with water.

But either will be fine!
As for the lintel - it will often be fine to just plaster over but if really smooth then just apply some PVA to it (mix a little sand to the PVA provides a gritty texture)

If using PVA then test the wall first for suction to see if you need another coat.

Paul