The subframes were originally zinc plated before they were painted, but like nuts and bolts and everything else, it doesn't last forever. The drawback with galvanising is that you'll have to remove the mounting bushes which is difficult and expensive - eight halves at £30 each! Powder coating would be the same, but tbh I don't rate powder coating for anything with sharp edges - it fails at the edges and then rusts worse than when it's painted because the powder coat flakes off in a big sheet.
IMO, the best thing to do is blast off the rust (or blast the whole thing with the bushes very well masked up) and then paint it with something like
POR-15 or Rustoleum. Both products are designed to kill any remaining rust and are designed for things like undercarriage. I used some Ronseal 'No Rust' semi-gloss exterior metal paint which does the same, then sprayed a few coats of satin black over the top for a factory-ish finish
