Before we start suggesting sanding out the runs and polishing we need to find out if it's a solid color single stage...if it's a metallic color...polishing after you sand out the runs is not possible.
If you prep the existing single stage paint using a block and 400 grit or 600 grit paper is fine...33Willys77 is correct with respect to taking a long time and keeping the car straight...if you don't use a block. If you know how to sand a car by way of properly using a block, the body work will stay straight. Also, if you sand through the SS color, (depending on what's underneath the Single Stage) and you don't spot prime, you could have lamination problems or ringing around the area you broke through...again...depending on what's underneath your single stage.
Hope this helps.