Twice this morning I've had to do this, so I guess that means its common enough that some of you could probably use it too. First I noticed my girl Sassy was licking her rump, maybe getting a hot spot, so I got out the spray can of Happy Jack skin balm to spray on it. I shook it well, then depressed the button on top but nothing happened. When I pulled the plastic button off, I could see that the stem had been leaking. It was almost full, but had lost all it's pressure. So I took it out to the shop, turned on the air compressor and with a nozzle pressed down over that little plastic stem aired the can up a little, which allowed it to spray again.
Then a couple of hours later, I needed to use a C clamp that had gotten left in the bed of my trucks through a rain which rusted the threads. When I got out the nearly full WD40, it wouldn't spray either, so I aired it up as well. Neither of these are likely to set up due to oxygen in my method, but I was thinking back to having done this to finish off a spray-paint can, and was thinking it probably would if I didn't empty the can.
Which led me to wonder if there was any kind of propellent on the market with some sort of coupling to connect the little plastic stems that wouldn't cause the paint to gel in the can? I guess this would be a wasted effort though, since it leaked out once, it would probably just leak down again, so I guess as a last resort this works and is free.