My go-to is a sandblaster or wire wheel, acid wash/neutrailize if needed, 2x wipe with wax and grease remover, wait 30 minutes, tack cloth and blow,
and shoot 2 or 3 coats of black epoxy primer, either PPG or SPI.

So far, I have not found anything better that the above.

As far as a product to apply to rust itself....
Recently I tried Rust Bullet on some inner roof structure and it is considerably more user friendly than POR. but 20 years from now
will be the time to do a review on it.

I hate POR15 and will never use it again/bad experience. A large area I used it on failed, leading to the sandblast/epoxy prime routine.