Is there a negative to using bedliner to coat the underside of the bed and possibly the frame?

 

I don't think so. Lots of guys spray that crap everywhere on everything. It's Tough ****! Protects everything.

I might just do that on mine someday.

 

I would say the bedliner products are tough as nails, but best applied to metal that doesn't have rust.

 

There won't be rust. It's pulled apart for welding / rust repair right now.

 

Frame if blasted and rust free, I epoxy prime and use Chassis Black. Underbody/wheel wells are usually not completely rust free, so I use a rust encapsulator and top with Raptorliner. It is a bedliner, but not rubberized and holds up to heat from exhaust. It's been five years since a ground up restoration and no signs of rust or peeling. It's a personal preference, but I don't like regular undercoating or products like Herculiner, Duplicolor bedliners.

 

If it's been prepped right so the bed liner paint will have good adherent.
If not right it will peel like any paint would in time.

Where as the rubberized undercoating adheres to a clean surface.

Orich

 

The thing about those coatings, is that they can also trap moisture in places if your not careful.
IT works good on a box, where there is a layer or rust protection on the steel already.
On bare metal, things could happen under the coating, especially if there is gaps for air or water to get in.
Others on here have had good luck with oil treatment products, they protect, can leech into tiny cracks, and can be reapplied. They protect well, and don't "fail", they just wash away slowly.

Box liner can not leech into tiny places and protect.