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How to Install MDF Crown Molding - Part 2

Step 3:

Make a drawing of the room and insert measurements, to the 1/16 of an inch, of each length of cornice molding on the drawing. It is important to have a drawing so that you can easily keep track of inside and outside corners for cutting purposes.

It is best if you have a helper to hold the end of the tape measure while you are taking the measurement.

Note: Measure each length at least twice. The woodworker’s adage; measure twice, cut once!

Do not cut the cornice molding yet!

Step 4:

In order to produce a professional looking job it is necessary to measure the angles at each of the inside and outside corners. It is highly unusual to find perfect 90 degree corners in a home even though they may look like 90 degree corners.

To obtain perfect joints the cornice molding must be cut so that the angles of the pieces of MDF meet precisely. This can only be done by taking an angular measurement of each corner angle. Measuring the angles precisely is best done with an angle finder as shown in Figure 9.

angle finder
Figure 9 - Angle finder

Step 5:

Cutting the MDF crown molding.

Use a sliding compound miter saw to cut the crown molding based on the lengths and angles that were determined in Steps 3 and 4.

Note: If you have not used a sliding compound miter saw you should make a few practice angle cuts, inside and outside angles if the room has both, so that you can get the feel of the saw and its operation.

Note: It is easiest to cut long pieces of cornice molding on the ground, rather than trying to use a bench.

Step 6:

Once you have a few pieces of crown molding cut, you should install them. Having a buddy to hold the end of longer pieces of crown molding makes the job much easier.

The pneumatic nailer makes pinning the crown molding to the wall and ceiling much easier than using a hammer. There are a few reasons.

  • With a pneumatic nailer you only need one hand to drive the nail, so one hand is free to keep the crown molding in position. If you use a hammer, you need one hand to hold the nail and the crown molding in position while you use the other hand to wield the hammer.
  • A pneumatic nailer will automatically set the nail head into the cornice molding about 1/16 inch. If you use a hammer you must go around and set the nail heads with a nail set beneath the outside surface of the crown molding. In most cases you will find that a pressure setting of 98 to 110 psi will work best for MDF molding. However, you should do some sample nailing to ensure that you have the correct pressure setting.
nailing crown molding
Figure 10 - Nailing crown molding

To pin the cornice molding to the wall and ceiling use a 15 to 18 gauge, 1 1/2 to 2 inch brad, spaced every 12 to 16 inches on the top and the bottom of the crown molding. Drive the brads at a 45 to 60 degree angle, relative to the crown molding, into the drywall or plaster that the crown molding rests against, as shown in Figure 10. Drive each nail in opposite direction to the last to ensure a strong pinning, even if you don’t hit a stud.

Depending on what style crown molding you have chosen you should try to place the brad in a crevice on the crown molding. This will ensure that it will be totally invisible when it is covered with filler and sanded.