I have taken steel M14 mags, Rem oil, or CLP, and old BDU Tshirts for rags, and simply rubbed them down until the surface rust disappeared.
A little elbow grease, and they are rust free.
If you have rust inside the mag body.........
1. Stick the mag in a padded vice, and apply just enough clamp to it that it wont move while you.....
2. take an old T-shirt/rag, and cut a strip of it long enough to slip it inside the mag body, grasping both ends afterwards with the middle area of the tshirt/rag soaked in oil.
3. Apply elbow grease, and check every once in awile with decent light to ensure you can see how your progressing.
Note: If it comes down to loosing finish to get rid of rust, then so be it.....as the rust has gotta go.
Have no idea if it's the quickest or easiest, but it's worked for me in the past.
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