Underseal

Epoxy chassis paint is about the best - IMHO.

Pete

POR15

SteveL

Put something more heavy-duty than just black waxoyl. POR15 or any of the various underbody stone-chip proof treatments. Then black waxoyl over that, if you want to really protect it.

ss

Go to a commercial (truck) factor and get some chassis paint. This is "any colour you like so long as it's black", because it's full of bitumen. This makes it flexible, as well as all that other good impermeable stuff that lesser paints can offer. OTOH, nothing else will go over it (they craze and crackle).

Most other paints, _especially_ Hammerite / Smoothrite, are poor under a body tub because they're too stiff and brittle.

You'll want a primer under it (whatever you choose), something that adheres well to bare steel. I'd use a zinc-based one, like Davids 182. Others might prefer an iron oxide primer, such as Finnegan's No 1, or POR 15.

Waxoyl is a good idea, but it goes on very last, over some good impermeable paint.

And for how long would you expect a waxoyl treatment to last - when should you "redo" it (and is there a pre-re-application clean-up process to be followed)?

I've used it straight over zinc rich primer to good effect. I reason that impermeable paint can create a barrier between the Waxoyl and any rust that manages to take hold. Dunno if this is correct or not, but there doesn't appear to be any difference yet between those areas I painted first and those I didn't.

In my experience the black version lasts well - 5 years so far on my car that's used every day. I tend to slap on a bit more if I'm underneath and notice it wearing thin where it's been rubbed off by my grassy drive. Cleaning off excess mud and allowing the car to dry is all that's needed.