I don't believe you actually remove it, but you can do some fancy fiberglass and bond work to blend it smoth. that would the easier way. The other way if you really wanted to remove it was to completely cut it off with a cut-off wheel, reweld with a tig or a mig, pressure test, clean and seal the inside tank, pressure test again, grind down the rough weld smooth, pressure test again for any pinholes in the welds, if all is good, then you can do the fancy work with the bondo and sand, sand, sand, sand, sand until smooth, primer, then repaint. The first option would basically just involve sanding down the paint to metal so that your bondo can adhere to something and then just do some fill in work with the bond and sand that until you achieve the perfect contour of the tank. Still a lot of work, but better than actually removing the tank lip.
