i just polished a tank last week.
the reason i say polished is because i didn't use sand paper(which is buffing). if you go to tap plastics, you can get a "#2" acrylic polisher that does just about everything for two bucks, then you can get a buffing wheel at an auto store. then you just apply the goo and use the wheel. this involves no sand paper, and is really cheap. the best part is that there is no damage that can be done to the tank. this will make any scratches look smooth and be invisible with water. however if you are picky, then you use some sand paper, start heavy and go fine, then you goo it up and polish. it is that simple. don't be fooled.
any more questions, just shoot me a pm