For a rocker panel and hidden un accessible body panel voids I would go with 3m cavity wax and their wand kit or something similar used in auto body repair. I just welded in new rockers in a Silverado and that is what I decided to use because I thought there is more research and OEM type approvals behind some of those products. The cost might be the only issue since the wand kit and each can is about 20 dollars each.