I would sand the tank down. You didn't mention what you painted with, but what I would do would be an epoxy primer after all your body work is done. Follow that with a High build primer surfacer and sand some more, you could then seal and topcoat.
If you want to get by cheaper, do your bodywork, then HB primer surfacer, then topcoat.
It all depends on what you aree looking for. The main thing is preparation, preparation, preparation......your paint will only come out as good as your...preparation.... Good luck...
Tom