I know this is an older post, but I have sprayed my truck with Fluid Film through a Shutz gun. Works well here in MI to keep the rust away during the winter and spring thaw. Inside of the bedsides, doors, rockers and the entire underside.
Also have driven 60 miles to Canada and had my previous truck sprayed by a "Krown" applicator. Google Krown undercoating and read up. The Canadians swear by the stuff. I found that the Krown product is no better than Fluid film which is available here in the states. Just my opinion though.
