I moved in a apartment building built around 1992. It currently have carpet for the flooring that installed by the builder. I am thinking to lay ceramic tiles in living room so I removed carpet. What I found underneath is concrete subfloor with some kind of white paint on it. I am not sure why the builder paint the subfloor before carpeting it? Most likely what is this white paint, is it some kind of primer? Can I use thin set on it directly, or should I use a paint stripper to remove it?