If you're already taking it apart, I would buy a seal kit. It probably got stuck by some of the paint leaking past some seals in the first place.
I've never worked on a Sharpe, but I know my DeVilbiss has a seal or two that requires some light oil. If the Sharpe does, only use a tiny bit, you don't want it getting into the air-stream and into the paint that you're shooting.