Hi All, I've been thinking about stripping our stairs back to bare wood. Stripped the gloss off one area of the stringer easily, but then hit what looks like a grey undercoat, which just sticks two fingers up at the heat gun. I can get it off - mostly but not completely clean - by wet sanding it (with a P3 mask on, as I don't know what it is!) but I'd like a quicker strip and cleaner result if I'm going to do the whole staircase. Anyone have a idea what this undercoat is likely to be and how best to remove it? Thanks!