something it might be - where are you painting? is it dusty in there? (if it\'s outside you can forget a smooth finish in britain at least, thpough come to think of it i haven\'t checked where you\'re from)
if your brush already has lumps in it then watering down the paint won\'t make much difference. also if the pallette is dusty, or if the cloth you clean your brushes with is prone to shedding fibres onto the brush (don\'t use toilet roll! just don\'t do it! kitchen roll works better)