Having followed several thread 'debates' about which 'chassis' finish to use (POR, powder coat, hammerite etc) I thought it worth mentioning a product that I have used extensively, and mentioned a long time ago on the Forum: Epoxymastic 121. It's produced by a company called Rustbuster (nr Spalding, Lincs) - just google 'rust' and you'll find the website. It is a 50/50 (hence 121 - one to one) mix that gives around 2 hours working time when mixed. It doesn't appear to be at all toxic, can be brushed or sprayed and can be applied to clean or rusty metal. The level of adhesion is very high - once cured it can only be removed by mechanical abrasion. They say it's about 6x stronger than other chassis finishes. The paint was originally developed for use on North Sea oil rigs, so deduce what you will from that!
Epoxymastic 121 comes in 5 colours, but can be overpainted with anything, and it also sticks like nothing else to the Alpine's fibreglass panel work too. You can stipple it as it cures to blend with existing underbody finish textures, and it can be over coated as many times as you like (but you are unlikely to need any more than a couple of coats).
I've used all sorts of finishes over the years for the more vulnerable underbody components, and not being a keen fan of powder coating (it does crack, and certainly doesn't last even though it looks good to start with), I prefer products I can use myself - nothing I've tried/used comes close to epoxymastic for efficacy.
I would hasten to add that I have no personal interest whatsoever in 'Rustbuster' - but the chap that runs the business is very pleasant, and always discounts well if you collect personally!
Hope some of you find this helpful.
PT