Here's how to stop the rust and prevent it from happening using something you throw away. Get an oil squirter like the one pictured and squirt your old used motor oil between the bed sides. Instead of throwing your oil away use it to stop the rust. You can squirt oil all the way to the front of the bed inside. Make sure the truck hasn't been driven on a wet road for at least 3 days before doing this. You don't want to trap water in there. You will have oil dripping when you do this so do it outside your garage. You will have to wipe oil off some places for a couple days but that's a lot better than having your truck rust apart. I also squirt it in the front fenders from under the hood, inside the doors through the door latch, into the tailgate and even remove the rubber plugs on the back side of the rocker panels and squirt it in there. If possible drive on a dirt road after you squirt it up to trap the oil in there.