HI,
I started off undersealing my van earlier this summer and still going!
Make sure you scrape all debris off first.
I used an aqueous degreaser first then some hammerite kurust, which seemed quite good.
I then used KBS rust sealer (rustshop.co.uk sell this) its evil stuff but bloody good when on the van and not your skin!!!!
I will top this off with waxoyl, others may disagree with this and use dinitrol or something. But i already have a can so will use it!
I would definately look at KBS though!
Luke