I'm doing a traditional 3-coat plaster over wooden lath to repair a 4×4 hole on an interior ceiling.
I'm using StructoLite base coat plaster.
I applied Plaster Weld to the lath prior to putting up the scratch coat, but I wondered if it is necessary or helpful to use the bonding agent between base coat and scratch coat, especially as it on the ceiling. I thought it might improve the bond, but I haven't seen a many references to doing this, and the product was invented after the heyday of 3-coat plaster systems.
Is it necessary to wet the scratch coat prior to putting up the base coat otherwise?
Best Answer
You shouldn't need a bonding agent, for putting a base coat on a scratch coat, or a finish coat on a base. However you do need to make sure you scratched the scratch coat properly, and you do need to dampen an existing coat of plaster before you put on the new wet plaster. If you don't dampen the existing plaster first, it will dry out before setting, and you'd have to dampen it afterwards and hope that it sets enough to be usable.
Just wanted to add, you didn't say how you were going to do the finish coat, but from experience you do NOT want to use gauging plaster and lime putty without adding some perlite fines. If you don't add the fines, on a lightweight aggregated base plaster, the finish coat will develop cracks upon drying. I don't believe this is a problem, if you are not using hydrated lime and gauging plaster for finish coat.