Crown moulding is installed between the ceiling and the top of the wall and is a great way to add to the beauty of a room. While crown moulding often comes as solid wood, you can also find crown moulding made out of medium density fiberboard, or MDF. MDF crown moulding is lighter than wood, doesn't swell in humid conditions and comes pre-primed for painting. Just like installing wood crown moulding, the hardest part of installing MDF is getting the corner pieces cut correctly.
Place your MDF crown moulding upside down on the base of your mitre saw. Place the top of the moulding flush with the base of the saw, and the back of the moulding flush with the fence of the saw.
Measure the wall on the left side of the inside corner. Position the crown moulding to the right of the blade. Measure from the right end of the moulding and make a pencil mark on the bottom of the moulding, which is currently pointing up, at the same distance as your wall measurement. Set the saw to 0 degrees and cut the MDF.
Measure the wall on the right side of the inside corner. Place a different piece of MDF crown moulding on the saw, aligned in the same way as the first piece. Position the moulding on the left side of the saw and mark your measurement from the left end of the moulding. Position your mitre saw to the left at 45 degrees, and make the cut.
Move the crown moulding piece you just cut to your table and position it so the cut end is hanging over the edge. Make a pencil mark along the top of the cut, just under where the face of the MDF is. This line will help you visualise your coping cut. Take your coping saw and cut away the bevel on the end of the crown moulding. This cut will allow this piece to fit flush against a non-bevelled piece of moulding. Be sure to follow your pencil mark as you cut.
Cut outside corners with the MDF moulding situated upside down, just as you did with the inside corners. For the right side of the outside corner, position the moulding to the right of the saw and set the saw to the left at 45 degrees. For the left side of the outside corner, position the moulding to the left side of the saw and set the saw to the right at 45 degrees.
Use a stud finder to locate the wall studs and ceiling joists. Place a piece of painter's tape on the ceiling and wall a few inches from the corner between the two areas, directly over the centre of each stud or joist.
Hole the first piece of MDF crown moulding in place between the ceiling and wall. Drill 1/16-inch pilot holes 1/2-inch in from the top and bottom of the moulding, wherever it intersects a stud or joist.
Place an 8d nail in one of the pilot holes and drive it in with a hammer. Once the nail is almost all the way in, place a nail set on the end of the nail and hit the nail set with the hammer. This process will allow you to countersink the nail without damaging the MDF moulding.
Apply wood filler over the nail holes and in any gaps in the corners. Scrape the filler with your finger to make it even with the rest of the MDF moulding, then sand it down flat by hand using a sanding block.