Do yourself a favor and go to Harbor Frieght & buy a 7" varible speed car polisher ($35.00) with the velcro wool pad ($7.00). You'll have the tank done in a few hours. As far as sanding, use a D/A sander with 1000 grit. Anything higher is just a waste of time. The polishing compounds will blast right through all the fine scratchs with the bigger polishing pad. Thrust me, You'll want to shoot yourself for not doing this earlier.