Some thoughts in no particular order;
- That swage has a crease in it; this is common on the first pass. A second pass at 90 degrees would have removed or greatly reduced the appearance of that crease. Clear evidence to me that it was only swaged once, gives me pause.
- The rig is 12 years old, most professional riggers will recommend 10 years and then replace. Its not uncommon for rigs to go 15 to 20 years in good weather conditions and with good maintenance; they recommend 10 to avoid liability and make more money. With that said, if I had the money I would replace just to be sure and its gotta help with insurance premiums.
- The staining does not 'look' bad, I've seen worse... but then again I've seen what 'looks' like something that was ok and then cut open the swage vertically and find a mess of hidden corrosion inside. Looks can be deceiving.
- Any cleaner that has phosphoric acid in it will remove that staining. I used a decaler/degreaser for cleaning stainless before I weld it to ensure super clean metal in the TIG process. Its liquid, so it would only work if you can loosen the turnbuckle and disconnect the stay, then dip it in a bucket overnight. My small 1/8 wire rigging on my 22 ft boat is cleaned by soaking it in a bucket (coiled up) and then fresh water rinsed. A gel would be ideal if you don't want to disconnect it.
- If you went with new rigging, I've seen Riggers dip the wire into 3M 5200 before inserting into the fitting, then swaging (twice) and wipe off the squeeze out. This is to prevent moisture from going down into the fitting.