You could seal the rust with a POR15 product. If your gate doesnt have any rigidity on the bottom area, then this may not be the best idea. But, if your tailgate is strong, with just a little rust. This is a cheap easy fix. Less than $50 and only a few minutes to install. No welding or drilling.
I totally agree, but if there is any blistering showing, the best way is to cut out the rust, seal, and then add your panel. Using double sides tape or any thing else will eventually fail as the rust grows. POR15 is a great sealer, but if the area isn't cleaned it will fail. This is definitely the least expensive way I have seen to attack the cosmetic problem, and is a good alternative to to buying a new tail gate. It will certainly give the user a few more years to think about it.
I could write a lot about how to permanently solve the tail gate rust problem, but that is more than I am willing to cover at this point. I know how to do it, but it requires a bit of work. There are a few design flaws, (perhaps deliberately designed by Ford to sell more vehicles), that can solve most all of the typical Bronco rust areas. I guess I should do a "How-To" on those, I'll give it some thought, it takes a bit of thought on how to put it into words that make sense to everyone
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The important thing is that this is definitely a good idea as a patch panel.
Good luck,