Wet plaster on aerated block

[SuperPav]

So I've read enough from people sh**ing on aerated blocks as terrible because nothing sticks to them etc. However, our upstairs is built from celcon lightweight blocks and thin joint mortar.

 

The plan originally was to just dot and dab boards on the walls, but before I commit, I thought I'd check, is wet-plastering (maybe with a sand-cement scratch coat) really that problematic on the lightweight blocks? I think I'd prefer wet plaster if possible...

 

The original parts of the house downstairs had some bits built from the rice-crispies style aerated block from around 1990 and that had a cement coat on it (more like render than plaster, with just a gypsum skim), and to be honest it was pretty solid with very few cracks or any issues.

 

So, have you successfully wet plastered any aerated blocks, and if so, would you do it again, and how?

[JohnMo]

I think if you wet it first with water, things stick much better.  Recently stuck stone slip to aerated blocks, it only worked if I wet the blocks.  Think they suck the moisture from the cement be too quickly, so it doesn't dry correctly, so the same would be true of plaster.

[PeterW]

Damp wall then Thistle Hardwall followed by skim coat (Either Multi or Pure) is the spec I have seen used on these blocks. Also seen a belt and braces of fibre mesh embedded around wall openings for reinforcement in the base layer. 

[Carrerahill]

1 hour ago, SuperPav said:

So I've read enough from people sh**ing on aerated blocks as terrible because nothing sticks to them etc. However, our upstairs is built from celcon lightweight blocks and thin joint mortar.

 

The plan originally was to just dot and dab boards on the walls, but before I commit, I thought I'd check, is wet-plastering (maybe with a sand-cement scratch coat) really that problematic on the lightweight blocks? I think I'd prefer wet plaster if possible...

 

The original parts of the house downstairs had some bits built from the rice-crispies style aerated block from around 1990 and that had a cement coat on it (more like render than plaster, with just a gypsum skim), and to be honest it was pretty solid with very few cracks or any issues.

 

So, have you successfully wet plastered any aerated blocks, and if so, would you do it again, and how?

I was on a site visit a few weeks ago and the guys were plastering aerated block, they had dampened the walls (maybe PVA was involved too) floor was all wet walls clearly damp. They were using Thistle Browning directly onto the block and had bags of multi on a pallet near where they were working. 

 

I think Thistle Hardwall would ensure adhesion as its designed for even higher suction backgrounds - but they seemed to use Browning.

 

Given they were about half way through a 3 storey care home and had 1000's m² to do, I reckon they knew what they were doing and it would not be falling off.

 

My plasters that I have used for the renovations and extension are good, I like them, I have often discussed plastering weird and wonderful substrates and things they have plastered before and they reckon they can make plaster stick to just about anything (sensible) you can throw at them and have proven to be right with some oddities in my house. 

 

Waterproof PVA I reckon would ensure you would get good adhesion. The mechanical key is there, it is just the suction that ruins the plaster bond as it dries it out too quickly.

 

 

 

Edited August 4, 2022 by Carrerahill