If you use any of the rusting solns that contain a mercury compound, the mercury will plate onto the gold (or copper, silver, brass..) metal inlays or part when it is applied.
The plating does not just rub off nor does carding remove it.
It has to be polished off which if on an inlay with cut detail can cause problems. It's also difficult to polish an inlay w/o disturbing the surrounding new blue finish.

Not the issue it used to be as Mercury has been pretty much elliminated from the rust blue solns.

It was mostly used in Express Blue solns (Quick Rust). Not so much that I can ever think of in Cold Rust solutions.

The Belgian Blue (Quick Rust) had it in it. Mercury Bichloride I think was the compound.
BirchWood Casey's quick barrel brown for MuzzleLoaders had it as well. That was a great quick rust blue formula.
Even the new Belgian Blue stuff Brownells came out with a few yrs back seemed to have it still.
I tried some of it and it plated out on warmed brass just as the old Herters Belgian Blue did,
B/Casey has since changed the formula to elliminate the mercury bichloride.

No problems with stuff like Mark Lee's Express soln's or Laurel Mtn slow rust. No mercury.
Sometimes the inlays will look a bit dis-colored after rust bluing with these. I think it's a bit of the copper in the copper sulfate in them.
L/M has it in the soln for sure. I think MarkLee does as well.
But that is cleaned up easily enough by going over the inlays with a very soft pink erasure. Doesn't damage the inlay or detail and will polish it nicely just don't over do it.
That discoloration comes right off.

The carding wheel can do some damage to the soft metal inlays. The wire impinges (sp?) into the metal and gives them a frosted look. Only further polishing can correct it and that is difficult
for the same reason as trying to remove mercury plating from them. Flush inlays are very difficult to recover to full brilliance especially fine lines on a finish blued surface.

Also the carding wheel can drag the soft gold and silver from the inlay and onto the surrounding steel surfaces.
Each carding cycle drags a minute amt of gold onto the surrounding steel. You really don't notice it untill the color starts to build. Then the golden halo appears and you are stuck polishing the halo away and starting over.
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Same reason you don't use a wire wheel on brass or copper that is intended for use on steel. It drags the softer metal around and deposits it onto the steel.

I stay away from the inlays with the carding wheel. Go around them with the wheel. Then come back with plain 0000 steel wool or just a piece of clean shot bag matr'l and card them carefully.