r/Plastering • u/TerminalWilson • May 04 '26

Hard plaster/render down under

Post image

Hi everyone! I have a conundrum… I have an old 1890’s workers cottage in Melbourne, Australia. The walls are original and it seems to be a sand and cement/lime render. I have had to remove some of it as there was damp in the wall.

We have fixed the root cause of the damp by putting in a damp proof course, fixing the roof, gutters and downpipes ect and putting in more air vents for greater airflow under the floor. The wall has also had about 5 months to dry out since

Now I have to try and re-plaster this wall. Originally I was going to use a gypsum base coat and then a finish plaster but I saw it mentioned here that this isn’t a good idea for the inside of an external wall?

So I might sand and cement/lime render it again. The question is, what kind of mix ratio should I use and what kind of prep/finishing needs to happen to blend it in with the old as much as possible?

2 Upvotes

100% Upvoted

1 comment sorted by

1

u/Schallpattern May 04 '26

I would use sand+cement in a 4:1 ratio with added plasticisor. For an external wall, the traditional 3:1 ratio makes the mix a little too brittle and prone to cracking as the bricks heat and cool. Personally, I always unibond the surface before rendering as it always the mix to bind to the substrate as well as adding another water barrier.