See the instructions below depending on what material letterbox you have
Powder-coat - Yes. Lightly sand the surface with a fine sandpaper to rough up the surface of the powdercoat. Remove all grease, dirt and debris from the surface and let the area dry. Apply a full coat of primer to allow your following final coats of paint to adhere.
Stainless Steel - We dont recommend painting over stainless steel.
Concrete - Yes. Mask the required areas. Remove all grease, dirt and debris from all surfaces. There is no need to prime the concrete piers, you can paint directly over the surface.