I want to coat the inside of my truck's box crossmembers with cavity wax. It looks like 3M is the best but it's pricey - especially since you should buy the application wands to apply it.
Has anyone used a less expensive brand with good results? I've seen SEM brand for less $$ but don't know if either is any good.
When I was working we used rp oil brand undercoating. Its a non-drip oil spray product and worked very well on our sand and salt trucks. Just before I retired rp oil was bought out by another company, but it looks like they still sell the same oil undercoating, but with a different name.
I'd like to stick with a cavity wax in those crossmembers as it seems the most effective (and stays put). I'll then apply an oil based undercoating yearly to the complete underside.
This video from 3M shows how it applies using a "clear frame rail'
Just make sure they are completely clean with no rust or moisture present. Applying cavity wax over damp rust will actually accelerate the problem.
I switched to using only the 'wet stuff' like Krown or Rust Check because if rust or dirt is present, the oil soaks through that right down to the steel, plus it creeps into the crevices between welded panels that typically wouldn't have much (or any) E-coat from the factory.
Anyhow, I know you are already aware of all that, but I figured it's worth saying nonetheless.
