Yes, you can apply DiamondFinish Clear over rusted metal to keep that vintage look and prevent the rust from spreading. You can even lightly sand the DiamondFinish Clear with steel wool 24 hours after the final coat to take some of the gloss away and give you a matte finish.
Clean the surface first with a KBS Klean solution (10 Parts Water and 1 Part KBS Klean). Rinse and let dry thoroughly. Then apply DiamondFinish Clear in thin and never heavy coats. For rusted metal, at least three coats are typically required.
Please check out the complete DiamondFinish Clear Application Directions before you begin your project.