With everything off the underside now, iv'e scraped and wire-brushed a few bits of surface corrosion that I can find (mercifully few) back to metal. Makita battery grinder with a brush - what a tool!. I'm going to spray the whole underneath (bar mounting plates, etc.) with white stone chip paint. But can I spray it straight on to metal, or do I need to prime the bared metal parts first, or use a rust remover/convertor?
You definitely need to apply something to the metal first. Seeing as you have gone this far, I'd go with an anti corrosion primer / paint like Owatrol, or Bilthamber Electrox/ hydrate 80 etc.
^^^ what Tristan says. There are better products that stone chip as well such as 2 part epoxy. Have a look at the Bolt Hamber website, it gives useful info about how to use what with what.