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.
Cortex brand fasteners work great for unpainted trim (available at lumberyards or amazon.com). They come in a kit that includes a bit, plugs and screws. All you have to do is drill the screw through the trim (the bit will stop the screw once the desired depth is reached), and then tap the plug into the hole flush with the trim surface.
Be sure to order the screws that are designed for the type of trim you're working with; that way the plugs will be made of that exact same product. And if you're working with textured trim, align the grain on the screws with the grain on the trim. A kit of 100 will secure 50 linear ft. of trim that's less than 10 in. wide.
Preassemble Window and Door Trim
Prebuild your PVC trim as shown and then install it as a unit rather than one piece at a time as you would with wood. This will take a little longer but will result in perfectly tight joints. The cement forms super-strong joints. So if you can clamp the parts together, you don't need screws at all. But in most situations, screws are a lot faster and easier than clamping.
Pocket screws are the best method of joining corners when you're cementing window and door trim assemblies (see next tip). Cut miters to fit and dry-fit your cuts to check the joints and lengths. Measure carefully when applying trim around vinyl- or aluminum-clad windows, and leave a 1/8-in. expansion gap between the trim and the window frame for caulk.
Use a Pocket Hole Jig for Corners
Predrill mitered corners with a pocket hole jig, apply glue, then join the corners with screws. Use the coarse-threaded version of pocket screws.
If You Decide to Paint
Although painting PVC trim isn't necessary, you may want to consider it, especially if you have a bunch of cut ends showing (some brands of PVC are protected from UV sunlight only on the outside). Edges that have been cut or routed could turn yellow over time. Cut edges also collect dust and aren't as easy to clean.
Before you paint, use mild detergent to clean off grime and oils left over from handling, and make sure the surfaces are dry. Until recently, you could only use lighter paint colors, because darker colors absorbed more of the sun's heat and increased expansion of the PVC, causing the trim to warp and the paint to peel. Sherwin-Williams is one paint manufacturer that now offers a line of vinyl-safe paints. There are more than 100 color choices, light and dark.
Fill Fastener Holes Before You Paint
Fill small holes with an exterior filler and lightly sand it smooth after it dries. Or use a paintable caulk and smooth it with your finger. Fill larger holes or damaged areas with auto body filler, and again, sand it after it cures.
Use the Proper Fasteners
Don't scrimp by using fasteners that won't last as long as the trim. Stainless steel trim screws are the best choice because they'll never corrode. You can also use hot-dipped galvanized nails, but they may corrode over time.
PVC expands and contracts with temperature changes, so fasten it well. Fasten to framing, never just into the sheathing. Select fastener lengths that will penetrate the framing by at least 1-1/2 in. No predrilling or countersinking is necessary if the temperature is higher than 40 degrees F. But lower temperatures call for both drilling and countersinking; otherwise the PVC may split. Place fasteners every 16 in. at both edges of the trim, spacing them about 1/2 in. from the edges. If you're using 10-in. or wider trim, add another fastener in the middle; and 16-in.-wide trim needs four screws every 16 in.
Rout Profiles with Carbide-Tipped Router Bits
You can easily rout decorative edges or grooves with routers or shapers, but use carbide-edged bits.