A craft I'm interested in is action figure customization. Basically, you can utilize new or old action figures and you take them apart and combine/mix-and-match them while also adding custom sculpts, paint, etc.
One problem I have with some of the joints I'm working with is that they simply won't move. Due to time on the shelf, and often paint application between the joints, they get so stiff that the force it takes to "break" the seal actually breaks the joint.
There's a hot water trick, where you can run the figure under hot running water or dip it in a bowl of hot water, but this process also softens the plastic which can lead to bending and stretching in ways that can be permanent.
I've also tried adding lubricant, such as mineral oil, to the joints, but some times it's so hard to swivel the joint to even get the lubricant into the space that I'm worried that process will break it just as well.
I'd prefer a permanent fix. Most of the frozen joints will end up re-used as-is (for instance, a frozen elbow, but the entire arm is being transferred to a new figure), so a solution to initially free up the joint and then steps necessary to keep it free.