If you have paint on a diamond plate, the best way to remove it is with a power washer. If you don't have access to a power washer, you can also use a garden hose and some elbow grease. First, wet down the area with the paint on it. Then, using a putty knife or another sharp object, scrape off as much of the paint as you can. Once most of the paint is gone, wash the area with soap and water to remove any remaining residue.