Sand/cement backing coat on thermalite blocks

by Carl
(Bath)

Hi Paul

I followed your video on how to apply a backing coat. However on thermalite blocks I encountered a lot of problems using sand/cement. (I appreciate that one video can't provide answers for plastering on all types of surfaces). I adjusted the method and eventually found one that worked for me. I would appreciate your thoughts on the changes I made, and if there are any other improvements that could be made (if there is a next time!)

The two mains problems I had were:
(1) The sand/cement mix dried out a little too quickly, despite me dousing the walls twice beforehand with a hose, and pressing in a thin coat first
(2) A 12mm coat had a tendency to fall off in sections when using the feather edge, seemingly because it was too thick/heavy

I applied these changes to my method:
(1) Applying a 5-6mm scratch coat (and letting it dry) and then a 5-6mm float coat. The scratch coat gave a much better key, and the thinner float coat never fell off
(2) Using waterproofer in the scratch coat improved the suction control ten-fold and gave a much improved working time (too long at times - I once had to wait until 1am to finish the float! :D
(3) Mixing the sand/cement by hand. This produced a much lighter and 'fluffier' mix than when I used a hand mixer, which was producing a much heavier mix

I will add that I'm very happy to have been able to render the room myself, and I wouldn't have been able to do it without your help - so my sincere thanks!

Carl

Answer
Hi Carl
If what you are doing has helped - which it sounds as though it has, then there would be no problem to carry on with this method.
From what you describe most of the problems would seem to be as a result of high suction. The scratch coat did not give any application problems probably due to the fact that you were not working it as long - it probably dried quickly but then the application process is a lot quicker.
This quicker application of the scratch coat even though it dried quickly was not disturbed with the straight edge during the drying process and so just stayed on the wall to provide a barrier for the next coat.
If you wanted to try a 1 stage process in the future - then either extra water with the hosepipe, or apply a couple of coats of diluted PVA (a milky consistency). This should control the suction and give you lots more working time - but if you're happy with the method you have found then stick with it!
The same applies for the mixing - use whichever method works but it does need to be thoroughly mixed - as you say to provide a 'fluffier' consistency. Normally the reverse will be true as by hand people tend not to mix it long enough! Plenty of plasticiser and plenty of mixing works every time!

Paul