I have a customer who states that some MDF crown and base boards that were installed on the job have shrunk a 1/4". They have told me that most of the gaps occur in the middle of the run and not in the corners? What is the best way to fix this problem? Is there anything that they can use to fill these gaps that will not cause a problem later if the material decides to expand?
(Cabinet and Millwork Installation Forum)
From contributor P:
I would first check to see if the crown is nailed securely since the gaps are in the middle of a run. MDF can droop or sag between nails if the nails don't catch wood when nailing, making it appear as if it has shrunk more than it actually has. I would check basically the same thing for the base. If it isn't nailed into the studs, it can move much more and much easier with differences in moisture.
Several years back, I started nailing MDF crown with hand nails rather than with a nail gun. I still use the gun in the corners to quickly secure the joints in place. But everywhere else, I nail by hand. Most all the crown we install is 5 1/4". I will drive a 16 penny finish nail through the center of the crown into the plate to draw it tight against the ceiling and wall simultaneously. I have not had a single problem with shrinkage since I have been nailing it this way, rather than bottom nailed to wall and top nailed to ceiling. And I always take the time to locate studs and nail the base to the stud rather than just cross nailing as I have seen numerous so called trim carpenters do. Once it is securely nailed in place, use a good quality adhesive caulk (not construction adhesive) along the edges. You shouldn't have any more problems.
Comment from contributor S:
When splicing a joint, instead of straight cutting (butt jointing) try angle cutting the joint at a 45 degree angle so that the face overlaps and use adhesive and a nail. This way the joint will never gap.