Just thought I'd throw a spanner in the works.
Only kidding - I agree with the above
Two points:
(1) Waxoyl will seep into tiny crevices - which includes slightly rusty surfaces - by capillary action (I personally recommend thinning it more than the recommended amount when spraying into cavities). This, unfortunately,
contradicts what your Bongo-friendly garage said about it being 'problematical if applied over rust' which I would find a bit worrying. Obviously, if the rust layer is thick and loose and it falls off, it'll take along with it whatever treatment that's on it! But, other than that, I can't think of a better material to use than 'wax'. (I painted the exposed suspension parts of my kit car with black paint and then gave it a light spray-over with thinned-waxoyl. After it dried, all mud and water just fell off it - it remained completely clean until the day I sold it. It's AMAZING to watch when you rinse your car off with a hose and absolutely not a drop stays behind on the treated surface.
(2) - and, I think,
much more important, you can underseal your car until the cows come home, but if you don't spray
inside your body cavities (ooh-er, misses) then you WILL end up with rusty arches, door panels and sills. End of.
It's Waxoyl for me, every time.
(Seriously consider buying a cheap 'airless' sprayer for around £25 and a couple of tins of 'black' Waxoyl (~£25 each) and DIYing it. There's a thread on this forum which has lots of details on how to approach this task. Your car will stink for a couple of weeks afterwards, but - boy - the sense of achievement is amazing!)