typically I prime the entire fuselage, and use either Krylon primer or the auto type. They have a good bite to the plastic and provide a good surface for the acrylic as well. Model Master also makes an enamel primer that works well but for me it didn't go on as smooth as the commercial types.
Also I have noticed that if I don't use light coats of Tamiya until covered it tends not to adhere as well (i.e. I spray it on too thick)
Good luck to ya!