I'll be putting a DIY bedliner on the INSIDE floor of the cab on my next vehicle (hopefully a 1st gen).
Don't like the idea of any sort of dried/mostly dried coating on the underside of any vehicle. In my experience it's more likely to promote rust rather than prevent it.
My current vehicle, a '95 GMC K2500 Suburban, is a perfect example of why NOT to use any sort of aftermarket coating on the underside.
Don't know if you're familiar with Ziebart, but it's a, supposedly, professionally applied under & interior panel rust preventative coating. The Sub has been Ziebarted but you wouldn't know it from the amount of rust on the frame & various parts of the body. The frame is especically bad. There was separation of the coating from the frame at practically every frame edge & consequently, literally, sheet rust everywhere! Still haven't got all of it off yet.
It was originally from Ohio, but I've bought other vehicles within a one year span of it, a '94 & a '95, that came from Michigan & West Virginia respectively, & neither one of those were aftermarket undercoated or as rusted.
Good luck.