Rust converter

I’ve used Ospho, no complaints. I don’t think there’s a lot of performance difference between the various brands. It’s a known chemical reaction.

POR-15 is the way to go. I have cast iron stuff that was painted 10 years ago, and the rust has never come back. It's a moisture cure urethane, and encapsulates the rust. All you have to do is knock off any flaking rust off. No need to grind, wire brush, etc.