ALL paint coatings have a major weakness, because when you finally close up a cavity such as a sill by welding, the paint is inevitably damaged, leaving bare metal, which will corrode.
The most satisfactory way of mitigating that is to use a zinc coating. Hot dip galvanising is best, but you can't dip a Mini shell so we can rule that out. Arc or plasma spray is next best, and CAN be done if you can get hold of the equipment. Last, by a very long way, is a zinc based primer. I would use the zinc primer if all else fails, and it can be painted over with as many coats as you wish of whatever paints system you are using.
The zinc primer is weak mechanically, unlike epoxy primer, but inside a cavity it is not subject to wear and tear so should survive fairly well. Zinc protects by sacrificial corrosion, even if scratched or damaged. But to cover all damaged areas and provide an extra barrier to keep water off the metal, a good cavity wax should be used, and reapplied regularly. It will also creep into the seams where the zinc will not go easily.
It is one of my ambitions to have a zinc arc spray machine one day, but they cost about £20k. However, the technology is basically that of a twin wire MIG welder with an added air jet, and bearing in mind that in the late 1960s the early automotive MIG welders cost the equivalent of £20k to £30k in today's money, I do have serious expectations of them coming down below £1000, the price of a good AC TIG. If the demand is there, someone will mass produce them cheaply.
Meanwhile I suggest the very best zinc rich primer that you can find, the weld-through variety where necessary, painted over with a good epoxy primer and any random colour of top coat that you have lying about. Then close up the welds, try to get some more zinc into the seams, and when it is all dry, and indeed only after painting the whole outside, inject plenty of wax.