You will certainly have to flat it back to put black on it. You may find you have to flat it back to bare if the chips are deep. Final finish 600 grit if there's still paint or if metal 400 and 2k prime, guise flat back to 600.
Nick
I dont think the blue is original paint as its black underneath. The chipping goes to the black, unless black is primer? But the under side is just black. I guess that means the blue needs to come off.
Fill with water and flap wheel? it is quite thick maybe 1mm paint.
I did buy a cheap lidl sand blast kit, dont know if that would work on a tank with my compressor.
Umm. I'd be tempted to take it back to metal if you want a decent job. Forget flapwheels, they leave track marks. I would try and get some starches stripper and get it all gone