Procedure for making LOS permanent?

HI all,

I treated some silver castings with LOS, followed by polishing (to
remove the oxidation from the surfaces I did not want black) but in
about a week, the polished surfaces were badly tarnished. Other
pieces which were not treated, still stayed bright and shiny, almost
like the LOS in the grooves is leaching into/onto the polished areas.
Is there something I need to to do fix or stop the oxidation process
of the LOS and to make it permanent and prevent it from spreading?

Angelo Giaimo