1200 after each primer but not after top coat. Some say dont bother after primer but do a little and feel the difference. Make your mind up from there but I did the 1200 after as it feels so much smoother and cleaner. 
firstly, you don't need to sand after every primer coat, only the last coat has dried
secondly, 1200 is a little too smooth, you can get problems with adhesion from the paint and it may lift, 1000 is about tops for wet sanding primer
thirdly, whoever says don't sand after priming are completes retards