old school bodywork method of pulling dents was drill small holes and use slide hammer with screw bit end to pull out dents. Typically using heat as you go. The holes also allowed the Bondo to grip better, but I wouldn't want it on my gas tank. Usually, they brazed the holes shut before filler applied. More modern method is to tack on slender rods on outside of tank and use slide hammer pulling on the rods. No holes needed for newer method.