Doesn't matter whether you use ply or MDF, but MDF is used mostly because it is perfectly flat, and a lot of the cheap ply is typically very warped, and just crappy to work with. If I were to use ply, it would be a decent grade that's nice and flat, not the cheap CDX usually used for sheeting roofs. On my builds I start with the strong back stationary in one spot, so it can be set up level, and perfectly square. After glassing the hull I put wheels on so it so the hull can be rolled outside for sanding. Mike