its to do with shrinkage of the plaster. as it dries it loses moisture, therefore volume, and shrinks. there are a number of reasons why this can happen though they all equate to the same thing
- incorrect ratio of water to plaster in the mix (ie too wet or too dry), most likely too weak
- background suction - if the moisture absorbs too quickly into brickwork.
- drying too fast (turn the heating off, keep a bowl of water in the room whilst it dries)
- incorrect choice of plaster. bonding is ok on brickwork, though browning is better and is usually the preferred choice. better still, a cement render.
it could be any one if not a combination of all of these things. did you pva seal the bricks first? on brick i use a strong pva mix, allow to dry then pva again and allow to go tacky before plastering.
if the plaster is sound, even if cracked, a tight coat over the top of it (with lots of pva first) should sort it out, no need to strip it off. or if its sound and reasonably flat then a skim should hold no problem (depending on the severity of the shrinkage)
one further thought, bonding coat will crack if you apply it too thickly as it loses a larger proportion of its volume through moisture loss. it should ideally be applied in layers no thicker than 11mm. if it requires more then build up in successive layers rather than trying to get the desired result in one hit.
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