Zippy,
Underseal is a pet hate of mine and your problem illustrates my point.
It does nothing to prevent rusting (a trade colleague insists it speeds the process up and will not have the stuff in his workshop) it flakes off, is a pain when you need to do some repairs and it makes the underside of a car look ugly.
As mentioned in the previous thread, IMO you cannot beat a decent coat of paint (this of course once the metal is clean and rust free) by all means one of the rust killing paints such as POR15 (follow the instuctions for overpainting) or "Rust Bullet" - has anyone tried that one? or at least a zinc rich coating.
In areas prone to stones, a good quality stone chip finish should be applied next followed by a nice glossy top coat.
That way the bottom of your car looks nice, can be washed and kept clean and you can spot potential problems before they become serious.
If you want belt and braces the whole underside can be given one of the thick wax treatments - this isn't a once and forget it either, it needs to be checked periodically and re-applied.
Underseal is good for nothing except covering up problems. It is evil and should be banished from the face of the planet
I'm not sitting on the fence on this one
