GMCFAN - if you have a worry about breaking through the rust you can use a ZR product called Prep Step. You just apply it, dry and wash off. It softens rust, cleans contaminates and solidifies the reamining rust, adjusts the Ph for maximum adhesion of Zero Rust. Prep Step is not an acid nor a conveter, it's biodegradable and it's available in powder or gel and it's cheap as borscht. You were wonder about ZR re: POR - I'd go out on a limb and say ZR is at least 1/2 the price of POR and a hellofa lot easier to use - no isocyanates and no lead to be concerned about.
Basically with ZR it will hold as well whatever it painted on ... if it's loose rust and the rust falls off ZR will still be on the part that falls off. It doesn't bind the rust to other rust.
