Here is a design that prevents drying-up of paint inside their cans when left in storage for long, the main reason for drying up being the interaction of air with the surface layer of paint...this being especially pronounced after some paint has been drained from the can during the course of normal usage.
The can has a sliding piston type floater with a ball-valve mechanism at the centre,which allows air out when the piston is pushed down, but not the other way. Paint can be taken out by simply pressing down on the piston-floater.