I would spray the whole tank. Doing only an area will leave overspray that is hard to "blend" into the paint job. I did this recently. Tape off the cap and petcock. Wipe the tank down with rubbing alcohol and then wet sand it lightly with 1200-1600 grit sandpaper. The paint will look hazy, that's OK. Wipe it down with alcohol again, let it dry, and then give it a couple/three light coats of clear. Took me less than 2 hours.