A friend of mine's roommate used oil paint to paint the entire apartment and did a terrible job of it. We have been stripping the oil paint off the walls to redo it all and in doing so, the brown layer of drywall is exposed with all the fuzzy fragments. We were told to lightly sand it and put two coats of primer on it before painting. Is there more to it than that or is that sufficient? I am attaching a picture of the walls.