I had an old can of poly in the basement, and decided to give it a try. Thin crust on top, poured kind of like egg whites. But after mixing with mineral spirits it seemed smooth, and the result on my new plywood workbench top is smooth and fully cured as far as I can tell.
Something similar happened to me. I stirred until my arm fell off and the poly-u worked out just fine.
I’ve also bought poly from BiMart (PNW hardware store) that was old af right off the shelf and needed restored. So much stirring.