Old stain not drying

I have several gallons of exterior oil-based stain, which I'd like to use on my deck.
I used it on a small section of railing, 2 coats, only to find that days later the surface still feels a tiny bit tacky, and one comes away with an oily feeling on one's hands.
The stain is about 6 years old, and was kept in an outdoor shed, in southern NY climate. The cans were well-sealed, and it appears that nothing was lost from them, but either some drying agent must be gone or some chemical change must have occurred.
Will the stain eventually dry by itself, or is there an drying ingredient I can add to it, or is the stain unusable?
TIA, Paul
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Yep, if they were frozen they aren't going to "dry".
Brigitte
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