I think now David you are into physical removal, going down the grades of wet and dry to bare metal and getting a nice grained finish to re-laquer.
In the old days I used to use Frigiline, put on with a camel hair brush and no over brushing. Put it on and stand back.
It is possible the plates you have may have been laquered with what was called, 'Dragons Blood' which hardens with age and is difficult to remove.
Good luck with it!
Clive