this thread wouldn't belong if it didn't get side tracked. LOL
MDF is pretty darn straight. Your really going to extremes, should look great. Damn stuff is tricky to work with. There is a product called dynaglass I used once , operative word is once. It was so hard when it dried I could hardly sand it. The speakers are only 9x20 so I am going to use very little filler ,mostly sand and check with a straight edge.

As for the finish , I think there will be a baffle board made of hardwood where the drivers are mounted and then possible some veneer down the middle of the sides with black paint the a bunch of coats of clear. Still not sure if I will chamfer the edges or round over or just leave them as hard edges.
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DOG is GOD spelled backwards.