SKLAR TIP: Try the Pencil Eraser Test to determine stain or rust.
The next time you see a brown-orange color appear on an instrument, follow this tip on how to tell if it’s rust or a stain: use a standard pencil eraser to determine the source of discoloration. Rub the eraser into the mark, and take note of what happens next.
If the discoloration is removed and the metal underneath shines with a smooth and clean look, the mark is only a stain. If a pit mark appears under the discoloration, then the mark is corrosion and/or rust which can be fixed with Sklar Polish.
Check out the Sklar Stain & Corrosion Troubleshooting Guide