Sealing, smoothing FG and MDF

when i build a box like yours i drench the edges of the fleece where they come in contact with the mdf. then i also do the inside. To reinforce that, I'll put smaller pieces of resin/mat along the inside edge to hold the wood to the fleece. Then I grind off the outer fleece, staples and all. If you need to go back and tidy up the edge, you can use duraglass and sand it so it's nice and straight. I usually do that last, once all my glass has been laid on the outside for strength.