I didnt have any luck with coating my tanks.
For 1 they are galvanized so most spray can paint wont stick unless you get a real good etching primer.
2-They are stamped with oil and it takes a lot of work to clean all the oil off because the rolling and stamping process actually creates micro-pores for lack of better terms which hold the oil.
3- most undercoating products will turn to complete gew-mess if they come in contact with diesel fuel.
If you want to put somthing on it find a really good industrial or automotive grade etching primer and prep the tank by spraying the hell out of it with brakeclean then get some wax/grease remover and wipe it with a wet towel and DRY IT WITH A DRY ONE. DONT JUST LET IT EVAPORATE.
Then put on a coating that is really tough. I was looking at this.Extreme Chassis Black® Satin 15 oz Aerosol
Thats my opinion, Or just find a plastic tank
BTW the new tank should have a new lock ring so dont feel bad about ripping out the old one, they rarely come out in one piece.
Best of luck
2008 f-350 CCSB 4x4 6.4 Lariat Spartan Tuned, MBRP Duals, S&B intake, egr delete, ccv delete,custom coolant bypass, RF amps, Infinity Component Spkrs, Pioneer Shallow Sub, 13.9sec best
1996 f-250 Extended cab Short bed 4x4 7.3 5-spd, D60, Diamond Eye4" duals, DP Tuner, Stg 1 Injectors, DIY Intake, Custom Regulated Return Fuel System, Adrenaline,billet ww,Comp 910 springs, D66 Turbo, 6.0 Intercooler, SD rear springs,Custom trac bars, Truetrac diff.