I thought all big kitcar namufacturers (Caterham, Westfield, etc) used to powdercoat their chassis??

In order of whats best and budget permitting imho.

1. powder coating, the stuff is the bollocks and have never seen a problem with it.
2. has to be 2k, tough as old boots and easy to apply.
3. hammerite, can look a bit messy if hand painted and splinters off easily.
4. red oxide with chassis black as a top coat. ok if you let it cure but its like polyurethane paint and you can get it off with a steam cleaner.
5. the good old airarsehole, all nice and shiny for a short while but its got no bollocks.

There has been talk of chip resistance but nobody has mentioned the rubberised underseal. You could always give the underside a splash of that , it would also deaden some of the buzzing from the tin work.

Bob