squirt them with water and brush what you can, for 2minutes and then walk away and come back and do the glass, do this once and when you come back again give it a good clean and look for faint black water lines coming out, if this happens, take a cloth and wipe just the vent bit free of water than can drip down,
when you come back a further time and you squirt it should be all nice and clean, you will get a few merky runs get away from you but what customer is going to complain about one window not being perfect?