I live in a bungalow, and I have a cupboard in my hall. In between the back of the cupboard and the kitchen there is a cavity leading to the roof. For many years I had an oil fired central heating boiler in the kitchen, I now have a gas boiler. The fumes from the old boiler went into the cavity, and there was no flue liner. I have used white emulsion on the the plaster inside of the cupboard a few times, and brown stains keep showing through the plaster. Is there a sealant I can use to stop these stains appearing. I only need a small amount, would Rust-Oleum Clear Satin Multi - surface sealer 0.25L from B & Q be ok.