Repair plaster to old walls
If the plaster to be repaired is more than approximately 5mm deep then a backing coat of bonding coat plaster will be necessary- otherwise it can be very difficult to get a flat finish free of cracks.
Sometimes rather than just skimming the patch - it may be quicker and result in a better finish if the entire wall was skimmed (meaning the recessing of the backing coat will not be necessary.
Once the patch is properly prepared the finish coat can be applied by following the six stages of skimming. (But these stages cannot be rushed so to repair plaster patch can take as long as a whole wall, due to the fact that each stage needs to firm up more than the previous.)
Wetting the edge of existing plaster will ensure any plaster accidentally applied will easily come off. Make sure you put the plaster where you want it - in the patch, and not where you don't - on the existing plaster.
It is important to ensure careful loading of your trowel to ensure not too much plaster is applied.
The first coat should be applied so that it brings the patch out to the same level as the existing, and no more. Always on this first stage just concentrate on flatness - not smoothness, that will come later.
Then when this has firmed up a little the 2nd coat will be responsible for a very thin covering to flatten the whole area by filling in any hollow areas left by the 1st coat or through shrinkage. The first coat should be firm enough that it will not be pushed out of shape.
During every stage it is important to keep cleaning the edge of the existing plaster with your brush and trowel.
Then the trowelling up stages can be completed - but not rushed. If you feel you're going round in circles it may be because the plaster is too wet and needs to firm up.
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