I've had a complete new central heating system installed, having agreed with the plumber that I'd do all the chasing & making good. So far, no problem. The radiator pipes are in, they're 15mm copper pipes, covered in a woolly sleeve. I'm going to fill in the holes with bonding plaster just short of the surface, then a top skim with Multi-finish, (just like I did after the electrical chases). My question is: can I put the bonding plaster straight onto the woolly stuff? Or do I have to put some sort of barrier layer in first? Thanks for any advice.