Welcome to Large Scale Modeller: The home of the large scale military model builder.
If you ever have a jar of paint, Mr Surfacer, etc. that you can't get open, just put it in the ultrasonic cleaner upside down. After a two or three minute cycle, the cap will come right off. I used to have this problem with Polly Scale (Polly-S - can't remember which was the last name they used) paints. Before I figured out the US cleaner trick, a couple of the glass bottles actually broke rather than the paint in the threads releasing the caps. That was back when I had some grip strength.