Without seeing it for myself, I'd have to say the diesel discolored the clearcoat on the tank, and that means the clearcoat will have to come off to get rid of the stain. I have the same problem on my side box, the aluminum is stained almost a copper color from diesel spilling on it. Since a nice pretty shiny polished aluminum box wouldn't match the rest of the truck, I leave it alone.
Check my lists of common part numbers in my photos! Updated often!
1991 F350 Lariat XLT, 7.3l, 5-speed, crew cab, SRW, 3.55, 2WD.
1990 Jeep Wrangler Sahara, in the process of a major overhaul/upgrade. Planned 4.7l stroker, spring-over-axle, interim 8.8" rear end till I get some 1-ton axles built & narrowed, and eventually a front shackle reversal.