This is going to be a short post: this is how I open the paint jars that are hopelessly stuck.
It happens with most paints which come in glass jars with metal or plastic tops: paint gets between the top and the glass, and glues the jar shut. I know I’ve picked an alternate color several times simply because I could not be bothered trying to pry a certain paint open. It’s quite prominent with Testors and Gunze paints, but Tamiya is not immune, either. (Tamiya has a better grip on the top, so it’s easier to force open.)
The solution is simple. Glass and metal/plastic have different thermal expansion coefficients. So we’ll use physics.
Get a small, shallow cup, and fill it with hot water to about 1-2cm deep. (The water should not be boiling; 60-70 degree Celsius should be fine.)
Put the paint jar into this water upside down. Only the cap needs to be submerged.
Wait for a minute or two.
Pick it up (carefully), turn it back (obviously), and using a piece of cloth, open it.
It’s as simple as that.
The only drawback is that this tends to remove the label as well, since you can’t make sure that nothing else but the cap will get wet during the process.