Coating back side of panels

[MASTERBrian]

I want to coat the back sides of my inner fenders, fenders, etc. At this time, I'm not looking for a show quality finish, this is just going to be a cruiser. For now I'm leaving the body in original patina form. I'll slowly get to the problem areas as time permits, but I want to drive her and enjoy in the meantime.

I have degreased the front section and I have metal prep as well. There are still areas with some rust and there's other areas that seem to have some sort of tar undercoating on them. What is my best course of action? I've thought POR-15 because it can go over the rust, but I worry, it'll be too hard to come back later and do any repairs or even remove bolts. Should I buy a good epoxy primer and spray it with that, or could I get by with some brush on rust-o-leum? At that point could/should I follow up with some sort of bed liner brush on/spray or what about the few cans of 3M rubberized undercoating?