Underside paint?

Any suggestions as to what is the best thing to paint the underside of my series three with?

Under body sealer. Bitumen sticky stuff. Best if you use the spay type. There a pistol there screws onto the can and then you can get in a round all the fiddly bits. If you're doing this, take note the cans need to be warmed up, flows better that way. But first get as much dirt and rust off as you can,,,, wire brush and paint scraper time. You'll need about 7 or 8 litres for the job, and don't be afraid to lay it on thick...

As in the chassis?
The dinitrol range of products are meant to be good.

I don't like underseals or bitumen type coverings, as there is a risk, indeed a likelihood, of water getting in between the coating and the chassis, perhaps where the coating has been damaged, and being trapped. If the coating looses adhesion in some place it can form a bridge over and hold water against the steel. This just accelerates corrosion. I prefer a good coat of a grease/oil mixture painted on. The oil allows it to soak in and the grease resists washing off. If you leave it to drain for a while before going on the road it does not provide a risk to motorcyclists.

I use Waxoil, thinned and sprayed on hot. Penetrates every nook and cranny. Three coats does it.

I prefer paint, as whilst the wax/oil coatings are good, I find myself under my Landrover often and I prefer not get oily muck all over me just crawling under them to work on stuff. I just recently sprayed mine with a couple of thick coats of polymer industrial floor paint which i'm really pleased with.

I recently used the oft-recommended Por15 chassis black, and am surprised by how flakey it is. Not a fan of the underseal types either. I've seen that trapping water when damaged.

I think that I'd say to galvanise if possible or waxoyl/dinitrol if not.