Plastering over different substrates

by Mark H

Our house suffered a damp issue in the early for of this year and the upper wall in the bedroom suffered quite badly. I have recently started to chip back the gypsum plaster to reveal a 50% lime (original) plastered wall and a cement base coat to the other side. As the property is an old stone cottage and the area cemented is in front of the fireplace, I don't really want to start hacking off cement renders and end up with the fireplace wall falling in. Can anyone advise which form of plaster would be the best to use to go over what is there... lime or gypsum? The leak problem has been permanently resolved by the way.

It's really difficult to say without seeing it - but you can't go wrong if you use lime!
The reason why? - it lets the wall breathe better than sand/cement. However that may not be an issue. Gypsum also does not stand up well to moisture - however if the damp problem is now ok then again this may not be an issue.
Lime is more flexible than sand & cement and lets the wall breath.

If you're just skimming only - then again lime will always be the better option if you understand it properly. However if you have no damp issues and are not worried about restoring as it would have been then gypsum may also be fine!
Draw backs of lime - takes longer to finish, being reliant upon suction of the wall / air drying / set with hydraulic lime. It can present problems due to shrinkage so needs consolidating as it dries.But the fact it can take longer to firm up means you can have all day or more to work on the same wall if the suction is controlled.
Gypsum does not give you as long to work it - max 2 hours but is more consistent and does not have shrinkage issues.


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