Patching in Thermalite

[EdTurtle]

Hi, I need to patch in some largeish areas on celcon blocks. The old browning had come loose in places, so has been removed back to the blocks. I cannot get hardwall here, and it has been said that bonding cant be used, so I'm doing it in sand and cement.

I had hoped to clarify a few things:

Should i PVA the walls first?
Should i use waterproofer or plasticiser in the mix?
The total depth is 16mm - should i do this in one coat, or do a thin scratch coat first, followed by the float coat the next day (although time constraints might mean this could be more than a few days - would this matter?)

I have only done limited work with sand and cement. Does the scratch coat have to be perfectly flat?

Thanks

 

[Bonni]

Yes to PVA or SBR. Theralites (lightweight blocks) is a high suction background. Applying PVA or SBR reduces it to a low suction background so it does sup the water out of your render.

Inside, I don't bother with waterproofer but I always add plasticiser, Optimix is the one I like to use.

Max 11mm for the first coat per City & Guilds. Top coat is often as flat as the background, so the flatter the background, the flatter the top coat.

 

[EdTurtle]

Thanks for that. I think the only bit that I'll struggle with is getting the backing flat - not enough room to get a featheredge in since its only patching. I can rule off the old wall for the top coat though, so hopefully that will make it flat.

 

[EdTurtle]

I have a few more questions if possible?

If i do one coat at 8-10mm then the next at 8-10mm will that be ok?
How long between coats - can i do one in the morning, one in the afternoon? Or, if i don't get round to it, one coat one day, and the top the next day - is better to leave less or more time?
Theoretically, if I didn't have any door linings etc, and it was just a straight wall, would one coat at 11mm work (as above 11mm max), or would it best to do two coats, say 8mm each?

Thanks

 

[Bonni]

I have a few more questions if possible?

If i do one coat at 8-10mm then the next at 8-10mm will that be ok?
How long between coats - can i do one in the morning, one in the afternoon? Or, if i don't get round to it, one coat one day, and the top the next day - is better to leave less or more time?
Theoretically, if I didn't have any door linings etc, and it was just a straight wall, would one coat at 11mm work (as above 11mm max), or would it best to do two coats, say 8mm each?

Thanks

Yes, you can build the coats up to the desired thickness, just kill the suction between coats with SBR.