I have simply seen more ridiculous finish applications from Guild members than all other woodworkers combined. Were it not for the incredibly forgiving nature of oil finishes they would be far more obvious.
French Polish is a finish system that has been well understood for centuries. How and why it works is pretty clear. Oil is used as a lubricant in regular French Polishing. The how and why are understood. Using just oil is not much different from the rubdown given to oil finishes after some curing has occured.
Sanding slurry finishing is the best example of a common guild headshaker. No one has ever shown me a test board with a slurry finish next to a straight finish that was not screamingly obvious.
I have read many systems from many guild members and know more than a few. I have never heard of any using an oil based French Polish.
Turning the cans of oil upside down is not the issue and does not stop the aging of oil. When oil starts to polymerize the linking is very gradual. It requires oxygen and generates heat in the process. When air enters the can it displaces the nitrogen blanket the finish makers usually put in the can.
It is impossible to stop the introduction of oxygen, so some manufacturers use a variety of "stablizers" to slow the linking. They act as oxygen scrubbers but need to evaporate fairly soon after swallowing the oxygen or they can do bad stuff to the finish.
Using old finish is one of the most common mistakes folks make and it can really bite them when the result is a gummy finish that will never solidify. Oil is too cheap to risk a stock finish on when there is no need...
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