Today I have been painting the underside of the 1990 Cooper which I am restoring.
A good friend of mine has a boat business and they do a lot of work on steel-hulled canal narrow boats. Every couple of years those boats are lifted and the steel has the barnacles scraped off and the steel hull re-painted with a special black bitumen-base paint. He gave me a few litres and I used it to paint the under-floor and inside of the wheel arches.
It went on very well and looks like really good quality weather-proof stuff.
I forgot to ask the trade name of this paint, but if anyone wants to do the same, just find a local marina which does work on canal boats and they may well sell you some.