To prevent the masking cap of mechanical parts from being blow off or polluting materials from leaking to the masking points during a high- temp. coating treatment.

A cap assembly 10 with a vent for masking the specific point of the parts by being fixed to the specific point has a closed end 11, an open end 12 and side walls 13. These side walls 13 extend between the ends 11 and 12 and form an internal cavity 14 of the specific point. A vent rack 15 extends along the side walls 13 in the cavity 14 and terminates at a pressure reservoir 17. A sealing and ventilation assembly 16 periodically discharges the air pressure in the pressure reservoir 17 when an excess air pressure is formed.