How was the old mdf attached? If nail and nail heads weren't flush, use the nail pattern to match. If screws, best if you first countersink so the screw heads aren't too proud or mushroom the panel. MDF doesn't give so screw head could split the edge, mushroom, strip the screw hole or the screw can snap off at the head. You also may want to put some blocks where the old and new edges meet so the joint is flush.
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