Before spending money, start off by trying toothpaste, rouge, face makeup, corn starch, chalk....any of that slurried in baby oil or water. Try hard texture cloths with that; finish with soft.
Failing that, try SimiChrome polish, available at any motorcycle shop, most auto parts stores, etc. They may also have plastic rubbing compounds and polishes made for plastics.
Rubbing alcohol will remove any trace of anything without leaving toxic residue.
In my opinion, if your tank is scratched so badly that it takes sandpaper to fix it, replace it or learn to live with it. Unless you have the professional products and procedures (like the guys on TANKED) working acrylic is a frustrating and stressfull undertaking.
The above is what I know from first-hand experience. Google Wiki-HOW and about.com for further advice.