Just laid sheet vinyl in my bathroom, a small job in prep for doing bigger kitchen job. I put down a good quality underlayment. From what I read on sites such as this one, you should patch the joints and nail indents and then sand smooth. The surprise came when I went to sand the patched areas. The patching compound had dried as hard as concrete (in fact I think it was concrete) and I couldnt sand it. It just wore away the grit on my paper with no noticeable effect on the patch. I was able to scrape and get it to the point where I could lay the vinyl. Before I do the kitchen, which is a much larger area, I want to make sure Im using the right patching material. It was Simple Prep Pre-Mixed Floor Patch. If thats the right stuff, how the heck do I sand it? Thanks.