I just finished rolling on the second coat of primer on the interior of my skiff. The building manual recommends three coats of primer then two coats of paint on top of that. The plywood seems very porus and really soaks up the paint, so I would say that that would be the minimum number of coats to seal everything and get a durable finish. I have been sanding between coats and cutting in by hand, leaving a quarter inch gap around the varnished areas. I'll cover that gap in the final two coats, which will be a darker "antique cream" color.