Depends on what type of seat it is and the finish on it i suppose, to get the deep scratches out use 600 then go down to 1200. I used 1200 on my one but it only had very very light marks, cant tell it been rubbed down at all.
For the shiny or smooth bits on the side i used 1200 and it went dull so i then gently polished it with the polishing mop and a cutting paste to make it shiny again then gave it a good going over with a plastic restore. its not turned out to bad can relly tell its been done.
hope it helps