This is a close-up as I was finishing applying the Zinsser BIN primer. I hope that this will show why so many woodworkers recommend sealing the cut ends of MDF. The dark lines are the factory sealed surfaces of the MDF donuts that make up the drum. The sealer obviously does not extend through to the mid-portion of the MDF. This part of the donuts fuzzed up and looked considerably more porous. 2 coats of a non-water based primer should seal it just fine.