Waxoyl do a great underseal, you can get it in cans to spray on or big
tubs to paint on if you like! it does say that you just need to wire
brush the loose stuff off, I used it on mine and have had no problems
at all underneath the car other than having to top the coat up every so
often.

If you want it to be concours then you will need to spray it with
stonechip or the like, I personally like to drive the thing and wish to
spend as little time lying underneath it as possible!

Miniman

I really like POR-15.  I would wire wheel the loose stuff and then use
the three steps that go with POR-15.  You can find a kit that should
work for you here: http://www.por15store.com/page/por15/PROD/Kits/SSK

That should take care of your problem.