I’ve never used a magnetic tumbler so I’m not sure if this is a
solution. I have used a rotary tumbler quite a bit and found
that I always had to change the solution after a half hour or so
to prevent oxidation. After I changed the solution once I could
let the tumbler run all night and not worry about opening a
barrel full of dark gray pieces. I never have found a way to
totaly remove the finish left in the recesses of the dirty
pieces, gold or silver. I always attributed the oxidation
problem to traces of investment still on the pieces when I put
them in the tumbler. Hope this helps.
Rick