After my YJ frame was repaired (patched), I used
Eastwood Co.'s Rust Encapsulator on the areas of the frame that had lost the factory black finish coating and were rusting. After the rust treatment, I painted the entire frame and the transmission crossmember with
Eastwood's Extreme Chassis Black Satin. When applied correctly, the Rust Encapsulator is supposed to be better than POR-15 at preventing a recurrance of rust. So far, I haven't had any problems. Just give any product you use plenty of time to dry in humid conditions.
As for that special paint technique where the steel is electrified first... I think you're talking about powdercoating. The positively-charged dry paint powder is attracted to the negatively-charged steel and a temporary bond is formed. The coated steel is then baked in an oven to melt the paint powder in place. The result is a very durable and attractive finish on the steel that lasts for years and can withstand a lot of abuse.