Strange they didn't know of it Dave. On this site they are advertising it post free ...
Rust-Oleum Stops Rust 18 oz. NeverWet Multi-Purpose Spray Kit-274232 at The Home Depot
(Boy you wanna see that link in full)
The site also helpfully shows you the nearest stockist. Not so helpful to me in UK, as it shows my nearest stockist to be in Cumberland. Hmmm? There's a Cumberland in UK ... I wonder? Nah, Cumberland is a county here, not a town.
I'll have to try ordering some on-line and test out if they will export and the delivery cost.
Regardless of other's hesitation, even if it works to simply waterproof a cardboard box as a litter tray, as in the video, I'm sure I can find a market for it over here. Hopefully it will prove usable on paper models though ...
Johnny.