To tap or not to tap?

[Ginger]

My guess is to remove as much surface rust as you can (via sanding or something) treat the rust with a killer (like naval jelly; hardware stores usually also sells kits from brand names) and then blast it with a durable high temp paint... though it probably won't last. Some kind of coating would be best, like jet hot, but that's probably more money than you want to spend.