The plasterer has just finished replastering my bathroom but has left about 1 sqaure foot where there is some plywood boxing-in. He said he couldn't successfully plaster onto plywood. I was hoping to have a paint-on-plaster finish to this area (not tiles). Surrounding plastered area is 4 to 5mm thick plaster, so it's too thin to nail on some plasterboard. Is there any way I can apply a 4/5mm skim of plaster to this plywood?