This is what wire looks like when it is on its last leg. You can see many spots where rust is occurring. The strands have lost their luster and appear dull and tarnished. Rust spots are less the rare occurrence and more the norm, as almost each visible strand has multiple rust spots on it.
It is important to note the strict difference between rust staining and rusting metal. If you take perfectly fine stainless steel and rest it up against a rusting piece of metal, the iron oxide from the rusting metal will stain your perfectly fine stainless steel. This means that if you have a low quality piece of metal attached to your rigging and it begins to rust, the metal of the stay near this will develop a rust color. Rust stains polish off and the wire will go back to looking like new.
If the wire itself is rusting, there is no amount of polishing that can be done to remove all the rust and restore the original luster of the wire. When the stay itself is rusting, that is when it is at the end of its lifespan.
When the stay begins to corrode like this, it should be replaced promptly. Yes, you can still sail in light conditions with it as it has now “failed” yet, but it will soon fail and should be replaced.
Failed rigging is when it actually breaks, and while you can sail with your rigging until that occurs, the problem is that when a failure occurs during use, the repairs tend to be rather costly!
Imagine for a moment that you have your sailboat sitting in the slip and the port cap shroud begins to develop significant corrosion. At this point, your sailboat is sitting in the slip with the mast standing straight and tall. Nothing has broken or given way yet.
You make arrangements and either replace the stay yourself or hire a rigger to replace the stay for you. Then you go sailing with your new stay and nothing happens.
Now lets imagine the same situation but instead of replacing the stay when it was dying, you wait for the stay to fully die. You are sailing along on a close reach on starboard tack. The spray is coming over the bow and you are heeling well to leeward. Everything is wonderful and then you tack. Now all that load is on the corroding port cap shroud and the failing stay finally fails. The wires break and the mast becomes unsupported. The cap shroud had broken and the only think holding the mast up in the air are the lower shrouds! The force of the wind on the main and headsail pull harshly against the top of the mast which is no longer being supported by the port cap shroud and the mast begins to bend. The mast bends further and further causing the sails to become baggy and hold even more wind, and pulling even harder on the unsupported mast until it buckles at the lower spreader attachment.
Now you have a broken mast as well as a failed port cap shroud!
This is why you want to replace your stays when they are failing instead of waiting for them to fail.