i personally would use underseal. very cheap and does the job brilliantly. can be brushed or sprayed on. cleaning, prepping, priming then painting is a ball ache undr a car. underseal will go over anything with minor prep work.
MOT tester is supposed to prod the structure with a special metal tool which will find 'nasties' under underseal! Sounds like your man just wants to do a quick visual check!
Hi
Sorry to butt in but when i took my van in for mot last year they moaned at
me for using underseal. they just said its horrible stuff and can be hiding many
nasties.
They passed it but advised me to take all underseal off for next mot
I don't like underseal or hammerite either- nasty stuff. In high impact areas (wheel arches, inside of wings etc) after rust repairs, prime, then use anti stone chip paint followed by primer and paint. For non impact areas, floor pan, etc, I like to use chassis black paint.- much nicer and cheaper than hammerite.
Then the whole underside gets a good coat of underbody wax- dinitrol for example.