| If your boxes are built properly you really shouldn’t need anything to seal your joints, that’s why we use glue, and screws. The glue is the only thing you need to create a seal between the 2 pieces of wood
mdf is a porous material already it will leak a bit of air no matter what, unless your fiberglass the box. But I am going to doubt there would be an audible difference if you were to compare 2 sealed boxes one with fiberglass insides and one with out
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