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Tips for Working with PVC Trim
Jan 26, 2018

What's the Deal With PVC Trim?

The PVC (polyvinyl chloride) trim we're talking about is also referred to as cellular PVC. It's PVC all the way through. Don't confuse it with high-density rigid polyurethane or PVC-coated products. While they too are highly durable and low maintenance, their installation techniques are different. PVC is a form of plastic that's used in a hundred different ways, including for plumbing pipes.

The trim comes in various thicknesses and widths, but it's most often sold in common sizes similar to other wood trim products. Some companies offer material that has an embossed wood grain side and a smooth side. There are dozens of profiles to choose from: bead board, skirt board, tongue and groove, quarter round, brick molding, coves, crowns, just to name a few. You can even get it in sheets like plywood.

The home center near you might stock only 8-ft. boards in a few of the most popular widths, so you'll likely have to special-order longer lengths and the specialty profiles and moldings. The price is the biggest downside to PVC—it's expensive.

Cement the Joints

One advantage of PVC is that you can “weld” joints to keep them tight and prevent water from penetrating behind the trim. Manufacturers recommend a special type of PVC cement that has a longer “open time” than the type of cement that plumbers use on plastic pipes. You can buy this cement wherever you buy the trim.

You'll have about five minutes of working time to clamp and fasten the joints before the cement sets. Smear a little cement on both surfaces and then clamp or screw the joint together. Wipe off any excess right away with a damp rag. Unlike PVC pipe cement, PVC trim cement is water soluble and won't melt finished surfaces if you remove it immediately.

Work it Like Wood

You can cut PVC products with the same power tools that you use for wood. But use only carbide-tipped saw blades; plain steel ones dull quickly. In general, the more teeth a blade has, the smoother the cut edges will be. Combination saw blades work well.

Sawn edges won't have the same shiny finish as factory edges, so if you can, plan your work so that cut edges will be hidden, and let the smoother factory edge show wherever possible.

Sanding isn't always necessary, especially if you plan to paint, but if you have a rough-cut edge near a highly visible area, use a random orbital sander with 100-grit paper. Belt sanding doesn't work well because the friction from the belt melts the plastic rather than smoothing it.

Scarf the Joints

Where ends of trim meet, join them just like wood: Create “scarf joints,” that is, overlapping 45-degree joints. Cut the first piece of trim to fall just past the center of a stud so the second, overlapping trim piece can be fastened to the center of the stud. And don't forget to apply cement to both pieces before securing them to the wall.

Bend it to Fit

One super-cool property of PVC is that it can be heated and then bent into any shape you can dream of. Once you figure out a system for heating the PVC, you'll find that trimming arched windows is a piece of cake—and kind of fun.

When heated to about 320 degrees F, the stuff turns into a wet noodle. Build a form out of plywood, and you can make consistent parts all day long. The heat blankets used by the guys who do a lot of bending are spendy ($1,000 to $3,000), but you could build your own out of a culvert and a torpedo heater.

Leave Expansion Gaps

PVC trim expands when it's hot and contracts when it's cold. As a general rule of thumb, if you're installing trim in temps higher than 80 degrees F, go ahead and fit joints tightly. If it's between 60 and 80 degrees, leave a 1/16-in. gap for every 18 ft. of length. Below 60 degrees, leave a 1/8-in. gap. Some products expand more than others, so follow the instructions with the specific product you're working with to be sure you leave enough room. After installation, cover the gap with paintable acrylic or polyurethane caulk. Avoid silicone-based caulks—they don't adhere well to vinyl.