Hello, I've just done a patch test on our stairs which have many layers of poorly applied paint, as I was looking into restoring them back to wood. Our house is built early 1950s for reference. While paint stripping, I found a couple layers of white emulsion, then a semi gloss cream coloured coat, then a strange mystery black coat, then at the very bottom brown varnish directly atop the wood. When I came to the black coat I thought at first it could be fire damage because it wasn't responding to the paint stripper very well, but eventually I was able to scrape it off. It had a strange smell to it as well. Would anyone know what this black layer is, is it just a coat of paint or some sort of treatment?