An orbital works best. Start with a 40 grit, then go back over it with 60 or 80 grit. Do not get it too smooth for the exterior or it may peel after staining. If there is a heavy finish on it you may have to use a side grinder with a 36 grit disc first. You can stain right away. You should also apply a borate treatment before staining. If you do, let it dry per the label before staining.