Are you using the standard little red disks, the fiberglass reinforced, or the metal? I think they're all called "reinforced". It does take some practice to not waste disks, the first time I tried ...

Sakamoto keeps an assortment of vintage fenders hanging around for demonstration and class practice. We needed a dent to fix, so Ken takes a few swings with a universal dolly at this Vee-dub fender.