Hi Flyte,
As roycv mentions, Acrylic varnishes are very good. I only use these now, because they are waterproof when dry, brushes wash out in water, give a good finish with several coats built up and they won't react with other older surfaces. It is just about the best finish for my Foam models and is easy to get in any DIY store at a reasonable price.
It makes for a good sealer over wood and will take other paints particularly acrylics and good quality 'toy' enamels like Humbrol over the top without problems. Most car body rattle cans are Acrylic now but good to test them out on scrap first.
It sounds like your friends 'Undercoat' stuff was actually 'Anti-climb' paint!!!or so old that all the solvent medium was evapoated. You must get rid of it all, maybe even resort to careful application of the Ronson Blow torch if necessary. Anything put over that will slide off it even if it does feel dry. A definate candidate for the bin.
(I recently noticed that woodworkers in the USA are blow-torching over twin pack epoxy resin finishes when still wet (On YouTube) to give a really glossy, streak and bubble free finish, might try that on something, not a boat though....Think Foam construction!!!)
Yep...go for the Acrylic varnish....Ronseal and similar are good and easy to get, their floor quality gloss is amazing. Remember its the whitish milky ones you want.
Cheers......Ron.