Some of the links on this forum allow SMF, at no cost to you, to earn a small commission when you click through and make a purchase. Let me know if you have any questions about this.
Etching primer is made to be used on bare clean metal. It is very thin when cured and you then would normally apply a regular primer and paint over that. It will not bond to old paint, but if it's clean bare metal it will form about as good a base as possible for the other layers to bond to. It's a combination of an etching acid (usually phosphoric acid) and zinc which form this base coat. It is often almost transparent with a slight tint to it.
I have no idea if they make a high temperature version or if heat would be a problem. Also the general consensus is the stuff in the rattle can is inferior to bulk can from a body shop. Something to do with the way the zinc & phosphorus atomize (or don't atomize) in the rattle can. But they sell a lot of it in rattle cans (Rust-o-leum, DupliColor, etc...).