You can use a low speed buffer at like 1000 rpm with a soft pad made for clear coats on cars and use the novus #3 and then #2. "scratches" wont buff out, they need to be sanded out first and then buffed out. If you can feel the scratches with your fingernail, you are in for a long road of sanding, lol.
I sanded and buffed a 3/4" thick 220 gallon envision tank that had a TON of scratches. Started at 400, and went all the way thru 8000 before switching to the novus products on a buffer. You can get sheets of 3M wet dry sandpaper from TAP that go to about 2000, you can cut these and use them in an orbital sander, but then you will need to use the micromesh smaller sheets from 2000 up. James from envision told me there are some places that sell the micromesh in full sheets, and if I ever had to do it again, I would shell out for a full sheet of each of the strengths and cut them to fit my orbital sander, sanding by hand can be a PIA! The Rockler woodworking store in Beaverton sells the smaller sheets of micromesh if you don't want to order a kit online.
I easily spent 50-100 hours restoring my tank to brand new and I would probably never do it again, lol...but I was a student during the time and didn't have the funds for a brand new tank.
Let me know if you need any tips... I sure did gain alot of them during the process...
Mike