The paint job is only as good as what it is sitting on. When it is sitting on top of an unstable product, it is unstable. Original lacquer paint on a 65 Mustang is not stable. Whatever they painted it with is peeling, which makes it unstable. You seem to be looking for a miracle product you can put over crap and make everything good. There is not such a product. As good as the SPI epoxy is, it can not correct what is under it. It needs a correctly prepared substrate to do what it does best. If this stuff was easy, nobody would pay me to do it. You will not get anything but the correct way to do something on this forum, even if it is not what you want to hear. Anything other than stripping that car to bare metal is half-assing it, period.