Plastering - When to use a scratch coat

[Porker]

I have a plastering job coming up for my sister which involves some surfaces I have not plastered onto before (slate and lime mortar). I got an answer from a previous post which helped regarding reducing suction with waterproofer.

I have always put the backing (sand and cement) on in one coat but I see that others use a scratch coat, then another coat of backing and the skim. When is it appropriate to use 2 coats of backing or is it just personal preference.

 

[jbonding]

there is little suction if any on slate it was used years ago as dpc to stop rising damp so you will have to form a key i would advise fixing eml then apply a scratch coat. theres a number of reasons for a scratch coat and when you would use one, firstly is to dub out and staighten a bad wall, the mixes between a scratch coat and top coat are different, your scratch coat is always mixed stronger than your top coat to reduce cracking, you may also apply waterproofer to your scratch coat depending on the job. doing sand and cement work in one coat the chances of it cracking and blowing off increases.

 

[Porker]

Thanks that all makes sense. I've always got away with one coat of sand & cement on brick but as the walls were all pretty flat it was quite thin. I will go with your recommendations on this job.