It's more of the primer or whatever you use sticking to the bare metal that is the issue. I like your idea of using Eastwood or Por-15 on the rust then Epoxy prime the floor. I put Truck bed liner on my interior floor and underside to seal it off. Not spray can bed liner but the better stuff that you mix the hardener in and use a kit that comes with the gun to spray it on with. Raptor is what I used, made in Canada and good stuff.