It is a noble iron oxide layer. The case is iron. You've to remove completly the old rustlayer and polish. Then completely renew. Note best antique browning on some days with multiple layers (may be hot at least for very dark) or "Fast browning" from the weapons business. The last method shows quick success with considerable results but it is not the optimum. Brass or gold plaqued parts of the watchcase don't react with this stuff. You can easy polisch or rubber it clean again.
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