I just finished installing MDF trim (1/2″ x 3″ with a stained birch back band). I used a nailer with 15ga nails. I used a chisel to flatten the MDF mushroom (held the chisel flat to the trim and sliced off the excess). Filled the holes with elmers lite wood filler. Sanded the filler and reapplied shellac primer over the the nails. When I painted the trim with semi-gloss latex I can see bulges at each nail (they look bigger than the original bulge that I cut off with the chisel). What did I do wrong? The wife now wants to change the trim color so I have the opportunity to fix the problem.
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If you smash up some MDF, you'll see that it seems to be fabricated in discrete layers, almost like sedimentary rock. Driving a nail between these planes naturally results in splitting.
I've only had marginal success with edge nailing MDF. But I can give you two suggestions. First, I think you need to use smaller fasteners (16g or 18g) and rely more on adhesive. Second, keep the nails away from the visible face of the MDF as much as possible.
An alternate solution would be to either shoot the 15g nails through the backband at an angle, or face nail, so that they would penetrate into framing as opposed to the MDF. However, I don't know if this would be possible in your particular case.
The last alternative would be to use solid wood instead of MDF for this particular application, which is what I generally try to do now.