I've been using Rust Encpsulator from Eastwood for large areas. POR15 was used in the battery box area of my 58 220S and it is holding up real fine. I painted all over my old Jeep pickup with rust encapsulator about 6 years ago. It held up fine and rusting "mostly" stopped. Heavily corroded areas did eventually rot through, but surface rust stopped dead. The new balck looks pretty good, I've used it on my 300D floors where I had some problems beginning. Doesn't cost an arm and a leg and from what I read, it can be sprayed with a reducer added. I do believe this rust encapsulator is something called Corroless by the OEM but I'm not positive about that.