Like Turbo says is it in the clear or in the base coat? If its in the base you will have to wet sand down to a smooth base and reclear. If its just in the clear wetsand untill smooth and buff, or if you go all the way through the clear to the base you will have to reclear. During painting if I make a mistake and make a run I use a peice of tape, dab it on the run and lifter it off, this will usually take the excess away and you can give it another coat n that spot and your good to go. If its already dry, razor blades do work if its a real nasty run then use wetsanding to smooth it. Let me know your problem speciafically and I'll tell you how to go about it.