rust prevention

I'm a firm believer in the POR-15. By encapsulating the rust-pitted areas it's supposed to prevent it from coming back. But I know that they want you to remove all of the scale that you can. I'm not sure that you could get by with using it without addressing it better first. If it was me, I'd try to do a cut and patch (with a lot of cleanup) before applying the POR-15. I can tell you that we've had great luck with it when we followed the instructions but had adhesion problems when we didn't.