yea you can keep filling and sanding BUT dont fill low spots of paint/metal with bondo or primer, feather the edges out, unless the metal is dented and you cant work the metal back out dont fill it, you can skim coat it later, if you have a flexable block that can conform to the shape of the tank with a flat surface , block that by hand, you can also use the edge of the block, i find that helps with a high radius panel.
I Use evercoat metal glaze, and if mixed right and folded, and Not stirred will produce great results with little pin holes, although there are always almost some there.

I would get the 3m powder guide coat and applicator, guide coat the whole tank and block that with 180 with a flexable block, then skim coat any low or imperfections, and block that out with the sanding block and 180 if that feels good then prime, and block with 320 -400, now primer is not a filler so dont try to use it as such, you have to get it stright with the glaze. -mike