Sand & Cement backing coat
I'm currently renovating my bathroom & have removed built in floor to ceiling storage that covered half length of a wall. The area left behind the removed storage is a mixture of exposed block and badly skimmed wall, but is all recessed between 10mm to 20mm from area of wall outside where the storage was. To level this im planning on applying backing coat to make level with rest of wall & then skim the whole wall. Can I use s&c over old skimmed plaster or is bonding coat more suitable? I understand bonding coat can be harder to level with a straight edge, so as a novice plasterer was wondering if s&c is the easier option if suitable.
A tricky one to answer! As you say Bonding Coat is harder for a novice to flatten but can be easier to build up this thickness due to the set helping you out. However there is no reason why Sand & cement cannot be used - just apply diluted PVA (water, pva and a handful of sand mixed in to give an extra key) to the existing plaster. You need not worry about the blockwork. Then I would aim to apply this thickness in 2 coats. Build out to approx half way and roughly flatten , then scratch up with a scratcher (a few nails protruding through a bit of wood) to ensure a good bond for the next coat. Then another day apply the 2nd coat.
If when doing the 1st coat it all seems to be drying due to reasonable suction then you may be able to build out all in 1 coat but if in doubt 2 coats will be safer.
Another method to cosider would be to stick plasterboard on to the recessed area with dri wall adhesive. If the depth is 10 - 20 mm you may just get away with it, but it can be hard to line it up perfectly to existing plaster in which case you may need to level out the odd area with Bonding Coat.
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