Exactly what the last man said waterbase is thinned about 10% only and coverage with a top quality paint isnt always great on some colours at the correct ratio so id be almost sure thats why its taking too many coats to cover, colour primers will help but even with a grey it shouldnt be that many coats. You cant wet flat waterbase coat as it will just kind of 'wash off' due to the water your using to flat it, wet flatting isnt reccomended on anything other than flatting and polishing anymore as you can trap moisture and risk problems with humidity blisters ect, much easier and cleaner to dry flat and or d/a it, best to just wipe it off using a waterbased degreaser if youve got a problem i find