close the container completely only if SWIY needs it to mix or something.. but leave it slightly open for venting when letting it stand like to wait for separation of layers or when SWIY is just not around.. If its just slightly open, the naphtha wont evaporate significantly but it makes sure that no pressure builds up. That being said, SWIM never really noticed any build up of pressure but he doesnt want to risk it so makes it as mentioned before