Hi Colin
I scrapped my underseal off using old wood chisels.
Then a knotted wheel on my angle grinder removed what was left followed by wiping with thinners.
This bit is very important, once you have the metal clean you have to scour it with 80 grit so the paint will bond.
I used 80 grit scouring pads like the ones used doing the dishes but from the paint shop.
Using the knotted wheel effectively polishes the metal after it has cleaned the underseal off so the scour abrades it again ready for the paint to stick.
The underseal on my car was a mix of old and new underseal.
Luckily I had a bit of practice scrapping underseal off my dads Lagonda Rapier a few months back so got to find the easiest way, funnily it was painted by the same person.,
You do end up with a lot of underseal flakes about the place and when using the knotted wheel make sure you are fully covered including a face shield as bits of wire fly about the place and I am still finding them everywhere.
Best of luck with you mission and remember to check the threads in the rear cross memebr and front outriggers before fitting the body.
Adrian