Not entirely true. You can use the stuff on bare metal providing you properly prepare the surface. (See their instructions)

APPLICATION OF POR-15® TO NEW METAL

For proper adhesion to aluminum, galvanized metal, and smooth steel surfaces, prep with POR-15® METAL-READYT Rust Remover/PrePrimer. Keep surface wet for 15 minutes, then rinse off with water and dry thoroughly. This process will leave a zinc phosphate coating on the metal and insure perfect adhesion.

 

I've heard of folks having issues with it not sticking well to bare clean metal, it may just be bad prep, who know's. Unfortunately my YJ was nothing but rust so I have no experience with it on new metal. I'm pretty sure it does say not to use it as a topcoat on other paints. I think if I was coing to use it on my JK I would use the chassie coat over the exsisting frame paint. Regardless, both the rust preventetive and chassie coat are definetly good stuff, hard as rock after they cure. As posted above, try to do it on a humid day, it cures much faster.

Al

 

Just had mine sprayed with a bedliner material, came out great. I had the wife's Durango done 7 years ago when it was new and still looks new underneath.

Factory undercoat is a joke compared to using bedliner.

3m sells a rubberized undercoater in spray can to do smaller areas.

Mark

07 Steel Blue Rubicon (undercoated)
Ketchikan, Alaska