As Stovens said the more rust the better, if those spots that you noticed the por peeling from didn't have any kind of surface rust i would imagine it probably didn't have anything to adhere to. It's really not meant to be used as an undercoating per say, but rather as rust inhibitor for particularly bad places. We use it on the under side of hoods and floors for the most part. Works excellent on a frame too but sand blasted and epoxy prime is best for that.