Cracks in plaster

by John Coupland
(Cornwall)

I've been renovating an old cornish cottage. One upstairs bedroom wall was very uneven, so I followed your guide relating to levelling the wall with a plastering sand mix. Afterwards, I keyed the mortar in a criss cross pattern. I left it about a week to dry out then plastered on top, and all went well. Now a few weeks on, the wall has cracks appearing following the criss cross key marks from the mortar underneath. Could this be caused by the mortar underneath still drying out? It has been very damp and cold for the last few months so drying may have been slow. If so, what should I do, If not, what do you suspect. Any help appreciated.

Answer

Hi John,

The backing coat plaster should as you say be scratched so as to provide a key for the skim coat but these scratches should not be too deep. Ideally a very light scratching of about 1mm is all that is needed - any deeper than this and the finish plaster may end up being too thick in these areas and if not properly pushed in and consolidated will be prone to cracking.
I think this could be a possibility in your case, however if the criss cross scratching was not very deep then if you provide me with more details regarding the backing coat mix, type of finish plaster, plaster thickness's and wall construction plus any other info you can then I'll try and work out what else could have caused this.

Paul