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. MikeFormer lurker, bought some plans yesterday and signed up on the forum. For the builders and experts ( and anyone else with an opinion), two questions: 1) Wondering if MDF if plywood is better for the stations? 2) Some builders have built their strong back on wheels? Are there any disadvantages to doing this vs building/securing to the floor?