I would say, from experience, stear well away form Hammerite on the underside of a car. It's far too brittle and cracks. And when i does crack, it falls off in sheets. I originally did mine in Tetrosol stone chip and Hammerite. I'm in the process of re-doing it all.

Seam sealer
Zinc or Etch primer on bare metal.
Primer
Re seam seal
Gravitex stone chip
2k Paint

I'm not bothering to laquer it.







Theres more pics in my rest thread.