What is this brown stain around the heat vent in a 1970's home that I recently acquired? Is it rust? This is on a lino floor. It doesn't have a significantly different texture from the surrounding linoleum. The lino doesn't seem any more brittle or damp or slimy where the stain is. It just seems to be discolored. Is it due to the heat? Is it benign? What is the best way to fix it?
It seems to have happened wherever there is a heat vent on lino. This is by far the worst case in the whole house. the others are much more subtle.