Use a hard flat backing piece behind the sand paper and the scratches and bumps will sand away flat.
Start at 120 grit and go up from there. Use water with a little dish soap in it as a lubricant when sanding. It will go slow at first, but the blade will start to come smooth and shiny as you get to 400 grit. Go as high as you like, but 1000 grit is usually good. If you don't like the look at 1000, , either go up to 2500, or start sanding back down the grits until you like the satin look. Often I go back dawn to 400 (1000,800,400) to get a nice satin hand sanded look.