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bedliner coating might be good if your frame is currently rust free and you do a lot of prep work and get total coverage. Otherwise i would be affraid that moisture and salt would eventually work its way under the plastic coating where you couldn't apply it. This would hold it there and keep it from drying causing more of a problem. On the upside, you wouldn't be able to see the rust forming...
I've been doing a lot of looking myself for Undercoating for my 02 which I plow snow with. We have used POR 15 in the past, and found that while it is a good product, it's best used as a base coat in combination with something else. Let me post up some of the links I found.
Look into a product called Fluid Film by Eureka chemical company. That stuff is excellent. It's one of the best penetrating oils I've ever used (not fast but if you have a few days and keep it soaked, it will break about anything loose) and it does a heck of a job as a rust preventer. It is made from lanolin, it doesn't really dry out and it stays where you spray it. You can get it in 5 gallon buckets and put it on with a wagner sprayer. A lot of the landscapers use it on their plow trucks and salt hoppers. I know some farmers who use it on fertilizer hoppers. I use it under my fenders, on my tailgate and inside my doors. It smells nice too. Be forewarned though, if you buy one spray can of it, you'll quickly run out. JD dealers carry it about $8 per can which is high. You can get it from McMaster Carr for 3 or 4 per can if you buy a case at a time.