yes, in reading about magnetism, feromagnetic, ferrimagnetic, antiferromagnetic and paramagnetic material [whew!], the only way sandblasting could have had any effect is if it heated the metal excessively, enough to warp it badly
the magnetic material on those magnetic signs is really weak at the best of times, good epoxy primer, surfacing primer and a heavy film thickness of topcoat could easily defeat them .... might mean the tailgate didn't get as good prep steps or as many coats
Bill
