One thing to be aware of is the bedding of the aluminum mast collar to the deck. The mast pushes against this thing on each tack and tends to tear any bedding you put there. If the bedding fails, water will leak under the collar and into the mast hole. From there, its a free ride through the plywood core and into your balsa deck. Even if you seal around the mast and keep water from leaking there, water can still get in there under the fitting. The only way to prevent this is to periodically pull the collar, clean it, and rebed it. If you never pull your mast, you’ll find it very difficult to do this.