How to Install Plastic Crown Molding

In each block, drill a 5/32-inch shank clearance hole through the tabs so that the threads of a #8x2-inch roundhead screw don't hang up on the plastic. Note that there are three styles of block: an inside corner block, an outside corner one, and a connector block. If you need to use a connector block, position it in the middle of a run or evenly space a series of connectors along a wall.

It's important to note that you don't absolutely need corner blocks when using plastic moldings. If you choose not to use them, note that the installation of plastic molding differs from that of wood molding. If you need to join two pieces of molding along a wall, use a butt joint, not a scarf joint, and apply adhesive at the seam. If necessary, touch up the joint with vinyl spackle and sand it smooth. Utilize butt joints at the ends of the molding. But eliminating the midrun block means that you need to create a temporary anchor point so you can spring the slightly overlong molding into place. This anchor point is nothing more complicated than a wood block temporarily screwed to the wall. For an extremely long molding run, leapfrog the position of the wood block along the wall to install each piece. Cut miters instead of copes for inside corners, and apply adhesive to the joint. If you sand through the factory-applied primer, touch it up with a compatible primer before painting.

Screw the blocks to the wall, making certain that they are square to the ceiling. Don't overdrive the screws or you'll risk breaking the plastic. To end a run, simply cut a tab off a block with a handsaw. Referring to the depth dimension supplied with the molding (or by measuring it), make a reference mark near each block and about every 4 feet along the run of the molding. The depth dimension is the height of the installed molding on the wall.

With your stud finder and strips of masking tape, mark the location of each stud on the wall. Measure between a pair of blocks and add 1/4 inch to ensure a snug fit; mark this dimension on a length of molding.

Cut the molding to length. A miter saw will do the job quickly, but a handsaw is just as effective. If you choose the manual route, make sure you cut straight across the molding and squarely through it.

To ensure a snug fit, apply the adhesive recommended by the manufacturer to the bedding edges of the molding. (The bedding edges are the molding surfaces that contact the wall and ceiling.) Don't overdo the adhesive or you'll have a mess when you install the molding.

Snap the molding into place between the blocks. Having a helper makes this part considerably easier because you need to ensure that the bottom edge of the molding meets the depth dimension marks you made earlier.

Secure the molding with a 2-inch trim-head screw driven into each stud. The slim square-drive trim screws usually don't require a pilot hole. You could drive finishing nails, but be aware that a misplaced hammer blow could smash details of the molding. Screws are far less risky.

Remove excess adhesive with a putty knife and mineral spirits. Fill the screw holes with a paintable caulk that's compatible with the molding. Also caulk any gaps along the wall and ceiling. Follow the manufacturer's paint recommendations.