[BRENT in 10-uh-C]

Are you certain it is rust forming? Typically the fins are made of brass or copper alloy. To my knowledge, those don't rust.

As far as cleaning it, I would probably start over and use EZ-Off oven cleaner to remove anything organic from the fins and tubes. This will include the paint which should take it down to the bare fins/tubes. If there is rust remaining, I would use either a Rust Mort or Rust911 product to remove that. Then use a Black lacquer to repaint the fins. A thin coating of paint is all that is necessary to prevent the alloy from corroding. Too much paint will act as an insulator, so just enough to get coverage is all that is necessary.