A masking device to prevent over-spray when using an aerosol can to apply aerosols to a work surface. The device includes a horizontal channel structure, the back end of which is essentially open and serves as an insertion point for an aerosol nozzle feed tube from an aerosol can, while the front end serves as an attachment seat for a masking unit. The back end of the horizontal channel is open, providing the user access to the aerosol nozzle in order to actuate the aerosol nozzle with a finger, whereby an aerosol is propelled forward into a masking unit. The masking unit has a small orifice creating a planar opening at the back, and a large orifice creating a generally planar opening at the front The masking unit is essentially hollow, providing the ability to contain an aerosol, the large orifice serving as a mask against a target surface. The masking unit has an observation cut out covered with a transparent film, a vent to exhaust excess pressure and aerosol, and raised beads radiating from the small orifice to the large orifice, which serve as excess aerosol accumulators.