An interesting read....
I have done 5 vans over the years with Protectacote and I thinks it's brilliant. All of the vans I have done have been ply-lined. I sweep the ply out, vacuum it, do a PVA glue wash to seal the wood, then run a bead of silicone sealer around the edges. Gutter sealer is good because it stays flexible and allows a bit of give with the ply without breaking the seal.
I always go 1ft up the side walls with the whole process too to make sure I get a good seal all the way around and the ply does not rot on the sides either.
I give it two coats, but I rotate 270 degrees, not the 90 mentioned above.
I give it a good scrub with one of my wfp's every 3 months or so when I'm parked on a decent slope so that the water runs straight out the back or side door. Every Christmas we have a 10 day shut down and I give it another two coats then. I swap my vans every 3 years, so whenever someone buys a van from me, it has either 6 or even 8 coats of Protectacote down. Because of the chemical composition of the stuff, it keys into itself meaning you can add another coat whenever you want and it blends in with the rest, not just sits on top.
I've never had a problem with rust, I do have the occasional connector pop off a DI vessel and flood the van, but I don't care because I KNOW there are no holes for the water to seep into, we just park on a hill, open the back doors and wash the road for free!
Until Protectacote lets me down, I wouldn't consider another option.