You can repressurize a spray paint can using an air compressor. The compressor should have at least the same PSI as the spray can. Remove the nozzle, attach the air compressor to the valve stem, and release a few short bursts of compressed air. That said, unless you have a reusable pressure container, you shouldn’t repressurize the spray paint can yourself. Most spray paints are stored in aerosol cans, which contain butane or another volatile hydrocarbon. These propellants are what provide the can with vapor pressure and propulsive pressure. Neither we nor our homes are equipped to safely repressurize these cans. It's always better to just buy another can.