I just inherited a project car (1966 Mustang Coupe) that has had the right rear quarter ground down to bare metal. I think the owner said he did this about a year ago before getting uninterested in the project and let it sit. The good thing is we are in Colorado, so not too humid and the car has been inside a garage (temperature controlled) since it was ground down. There is no visible rust or flash/surface rust in these areas that I can see, but I don’t want to take a chance and have something pop up down the road. What is the the best remedy to make sure any potential rust in this exposed area is taken care of and then I plan on laying down a coat of epoxy primer and go from there with body work, etc.