I do autobody work and have used featherlite wich is an inexpensive filler the same company makes a filler called rage gold.
IT IS DAMN STRONG STUFF. And it can fill some depth. 1/4 '' is the ''maximum" you should fill but it will do more. And the 3M glazeing putty is also handy stuff and can be sanded super smooth. After I finish the filler to 80 grit I skim coat the entire area making sure to work the glazing into all the scratches and work it down with 180 grit. Stay away from BONDO brand products,they will frustrate the crap out of you. On a side note are you having someone coat the inside of Your tank? If so get that done before the filler work. Good luck