I just remembered I was going to post up what I use for undercoating for Patrick and a couple others.
Its Amsoil Heavy Duty Metal Protector. It comes in spray cans and is like a very thick wax. Rocks don't seem to be able to get through it and it hasn't broken down even on the 90* days.
http://www.amsoil.com/shop/by-produ...tants/heavy-duty-metal-protector/?GroupID=200
You can buy it cheaper than that from other places but it gives you an idea. It goes on fairly clear with a very slight yellow tint unless you put it on really thick (like a run or drip) and then it turns a brownish color. I think I used 3 cans for my car last year. I'll probably order up 2-3 again this year just to be safe.