If this final step before topcoating sounds like a lot of work, you're right. However, Ferre says this is the most important step in refinishing a car. "It's your last chance to get the car straight before you paint it." To get it right, sand until the guidecoat is gone, constantly checking your progress as you go. When you get to the point where you just start to see the layer underneath the primer (in this case, the gray sealer), you're done.