Underbody spray on protection

I have a 97 SWB Ford Transit which is a little fond of rusting. I've made some repairs to the inner sills and got it through the DOE. I'm now looking for a good product/s to for the under carriage.
It needs to serve 2 purposes:
1. Be thick enough to hide my workmanship (Riveted and welded plates on sills)
2. Protect or at least delay further rusting

I'm thinking of Schultz but maybe I could do something else before I apply this??
Also what sort of prepping would the area need?

Any info would be greatly appreciated.

Realise this is 3 months old but theres no substitute for spending the odd hour under the van cleaning and touching up with paint here and there! I'm about to take each wheel off off one by one and get each corner as new as possible and paint the towbar in hammerite while i'm at it!

Funny how things jog the memory.
Waxoyl has been around for ever I can remember using the stuff about 1967 on a 1958 Beetle

However, I now would be inclined to do a test area if using a number of different underseals.
Was helping with strengthening a pick up frame this week, the original frame was steam washed etc. When the new supports were added on Tuesday they were cleaned and the frame wiped down with thinners. On Wednesday a coat of red oxide paint was applied to the entire frame and yesterday an attempt to stone chip ended in disaster because it reacted extremely badly and blistered back within a minute or two. We steam washed the residue off but the red oxide had become soft again and thankfully most of this has come off too, which will save on sanding.
I hate it when you end up doing the same job twice