Nitric Acid is by far the best thing to remove SDF stain, however, it is very corrosive to some surfaces and skin so use extreme caution.  You can get it at hardware stores such as Home Depot in the form of Concrete Cleaner solution. It will eat metals, marble, and remove the surface luster from some artificial surfaces, but we’ve seen it safely used on plastics, formica, corian, granite and other surfaces. You may also try D-Lead soap (order it on line).  It’s  a soap for firearms users to remove lead residue from skin and will also lift silver.  I have not tested it so I cannot vouch for its effectiveness.