I've done a rounded off external corner on a stud wall out of bounding on EML. It was to be coated with 3mm of multi-finish (prior to another skim when the plasterer comes). However, the profile dips in a bit soon from the tangent on one side, where it will need to up 10mm to reach the desired profile. I had got multi-finish for this coat. Will this be OK for up to 10mm (mixed quite thick), or should I go and get some one-coat stuff (I presume this will still provide a smoothish finish). Cheers, Simon.
It was to be coated with 3mm of multi-finish (prior to another skim when the plasterer comes).
will need to up 10mm to reach the desired profile.
quite thick), or should I go and get some one-coat stuff (I presume this will still provide a smoothish finish).
No, finish coat plaster shrinks a lot as it sets. You use a thin coat so it just gets thinner by shrinkage. If you put a thick coat on, it will crack across.
What you can do (I do it occasionally) is to make some bonding coat out of finish coat, by mixing a little PVA in with the plaster and as you say, mixing it up thicker. This can be used under your finish coat, but not as the finish coat. It will still crack across, but being bonded, it should stay in place, and the cracks provide extra key for the finish coat. I tend to use it to fill plaster chases and other holes, which I skim over when it's set, and it means having only one bag of plaster (and it can be old - past the use-by date). I don't know how well this would work on an external corner.
Finish coat should ideally be applied when the coat beneath has just set and is still wet. If it's dried, you will have to paint with diluted PVA it as for a reskim.
Well in the end I'd got some Wickes one-coat plaster and it was fine. For some reason it is white rather than pink/brown, but it looks rather like bonding and finish coat mixed in texture and when applied. I was not after a particularly smooth finish (will be skimmed over by the proper plasterer), so did not smooth or polish after I had run the radiused corner. Simon.
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