After emerging from the machine coater, the primed wood is dried. Latex primer can be dried under controlled conditions in 15 to 30 minutes - much faster than oil-based primer, which spends 12 to 24 hours in special drying rooms (Figure 2).

Figure 2. After leaving the machine coater, the siding is placed on racks to dry.

Prepriming Advantages

Clearly, one advantage of using preprimed siding is that there is no need to wait for good weather to get a protective coat of primer on the boards (Figure 3).

Figure 3. Preprimed or prefinished siding can be installed at any time of year, since proper painting conditions are of no concern.

Fans of preprimed siding tout other advantages as well: The siding's moisture content can be controlled; all four sides of a board can be coated; and the coating can be applied at a consistent thickness.

Moisture content.

At a job site, the moisture content of unprimed siding varies. Especially in areas with frequent spells of cool, damp weather, painters are often tempted to work in less than ideal conditions. "We have to be careful. We have days when in the morning it's 45 degrees outside, and it can be 110 by the end of the day," says Jesse Head, who appreciates factory-primed siding. "At least with prepriming, it's been done under controlled conditions." When siding is factory primed by a reputable company, the siding is usually tested for moisture content. For example, Cape Cod Siding in Nova Scotia, kiln dries their siding to a moisture content of 12% to 14% before priming. However, smaller companies may not be so careful. In one instance, a factory finisher reported "occasionally" checking for moisture content in siding before machine priming.

Priming all four sides.

If siding is nailed up unfinished, then only the front face of the siding can be coated with paint. "To do it right, the wood must be totally encapsulated - front, sides and back," says John Lahey, president of Fine Paints of Europe. Otherwise the back face of the siding will pick up moisture from the sheathing, which can be dampened by condensation from the building's interior, or from wind-driven rain. Factory-primed siding is encapsulated on four sides - at least until the first cut is made. When installing preprimed siding, a can of paint and a brush should always be on hand. "That's the largest problem that plagues us - installers forget that when the product is cut, the ends need to be sealed," says Scott Babbitt, eastern market manager at Olympic Factory Finish Group. "Unfortunately, a majority of the time it isn't done. Unsealed cuts tend to absorb moisture. That can lead to tannin bleed and premature failure of the siding." Manufacturers of fiber-cement siding point out that unlike wood siding, fiber-cement doesn't need to be encapsulated. "The nature of the substrate doesn't require back priming," says John Dybsky of James Hardie Co.

Coating consistency and coating thickness.

Fans of factory priming appreciate the evenness of machine application. "We are able to get more consistent rates of application using a machine than you can with a brush," says Dan Landry of Cape Cod Siding. On the other hand, just because a coating is consistent, doesn't mean it is thick enough. The major manufacturers of preprimed siding claim they achieve adequate thickness through a stringent quality control program. This includes machine measurements of paint thickness. However, not all factory finishers are scrupulous, and less reputable finishers have been known to thin the primer. "There is a tendency on the part of the preprimer to choose a paint that goes through the machine easiest and dries the quickest," says John Lahey. Mark Knaebe at the Forest Product Laboratory agrees. "I've seen some lousy stuff coming out," he says. "Some of it has only half a mil of primer. It should be a minimum of one and a half mils." For adequate priming, according to Knaebe, none of the wood grain should be visible through the primer, which should form a hard film with no noticeable chalkiness. Most experts recommend that 6 mils of wet primer be applied to achieve a dry film thickness of 1.5 to 2 mils.

Who's Doing the Prepriming?

When you buy preprimed siding at the lumberyard, it isn't always clear whether it was primed by the siding manufacturer or by a so-called "prestaining" company. Some brand-name siding is factory primed by the siding manufacturer (for example, lodgepole pine siding from Cape Cod Siding in Nova Scotia, or StepSaver cedar siding from Skookum Lumber Co. in Olympia, Wash.). These larger companies are likely to have well established quality control programs. Increasingly, lumberyards purchase unprimed siding and contract with a local priming company. Sometimes this is done in large lots, for inventory. Some lumberyards can even arrange for custom priming to meet a builder's needs. The turnaround time for a custom order varies. In parts of the country where construction is booming, the typical three-week turnaround time for custom priming has recently stretched to up to eight weeks.

Cost

If you are convinced of the advantages of factory priming, you're probably ready to ask how much it costs. In most cases, a factory can prime siding cheaper than you can, making the purchasing decision a no-brainer. "Factory priming costs one-third the cost of doing it ," says Tucker Smith from Skookum Lumber. Mark Knaebe agrees. "It can be painted cheaper in the factory, and you don't get the problem with sunlight," he says. The cost of preprimed wood siding varies, of course, depending on the species, profile, and grade of siding. Dan Landry of Cape Cod Siding says that their preprimed lodgepole pine siding "would cost the contractor, depending on the profile used, in the neighborhood $2.00 to $2.50 a square foot."

Topcoat Available, Too

Some siding manufacturers or distributors offer siding that is not only preprimed, but is also prefinished with a topcoat. "Seven years ago, we expanded our business to include two-coat work," says Fred Churchill of Churchill Coatings. "We can put on one primer and one finish. The extra coat only adds about $400 for 3,000 square feet of siding, compared to an $800 first coat." Prefinishing saves contractors from braving the inclement weather in the Northwest, the Northeast, and the Midwest, the strongest markets for manufacturers of preprimed siding. "The demand is great in the winter," says Churchill. "When you buy a preprimed product and put it up, you have to get that finish coat up in 60 days. Sometimes you can't do it in 60 days in New England. When you buy preprimed and finished siding, you can wait until next spring." Several manufacturers, including Maibec Industries in Quebec, offer factory-stained cedar shingles. Each kiln-dried shingle is dipped in an acrylic stain, dried, and boxed for shipment. "The use of stained shingles has doubled every year over the last three years," says Danny Rouleau, sales manager for Maibec. "Thirty percent of our production is stained or bleached." Some manufacturers of prefinished siding offer impressive paint warranties. For example, Cape Cod Siding guarantees the coating to stay on the wood for 15 years. "If the coating fails within the first 5 years, we supply the labor and materials to fix it," Landry says. "Between 5 and 15 years, we would supply the coating." New Jersey builder Matt Porraro usually buys siding preprimed and prefinished. "I know when it's shipped, it's protected," he says. Through his local lumberyard, he requests two coats, and tops the boards with a third coat after installation. He has a simple solution when customers don't think their siding needs prefinishing: He won't guarantee the finish. Not every builder who has tried prefinished siding is a fan. Bob Lipovsky sticks with preprimed siding rather than prefinished. "You end up putting nails into it and you end up painting it anyway," he explains. Scott Babbitt of Olympic notes that some installers have unrealistic expectations for prefinished siding. "It's a problem - the perception that prefinished siding can be nailed up and you can walk away," he says. "We're not applying vinyl siding here, guys." In most cases, prefinished siding will need a third coat of paint after installation, if only to cover caulk and nail heads. "Some colors - particularly the whites and the yellows - don't lend themselves to two coats," says Babbitt. "They require a field-applied third coat, partly because of the handling and the dust on the job site."

Ordering Preprimed Siding

Factory priming seems to be the way to go, with one important qualification - it has to be done right. If you're ready to try preprimed siding, ask your supplier to identify who does the priming. If you get assurances that the finisher checks the moisture content of the siding before running it through the machine, uses a quality paint, and can verify a minimum dry film thickness of 1.5 mils, then it might be time to try some preprimed siding. Marie Tupot Stock, is a New York City based construction and design writer, and executive editor of Tile & Decorative Surfacesand Fenestrationmagazines.  

Sources of Supply
Cabot Stains 100 Hale St. Newburyport, MA 01950 508/465-1900 800/877-8246 Manufacturer of primer and stains for machine finishers	Granville Manufacturing Co. P.O. Box 15 Granville, VT 05747 802/767-4747 www.woodsiding.com Preprimed and prefinished quartersawn clapboard sidings
Canton Lumber Co. 9110 83rd Ave. North Brooklyn Park, MN 55445-2197 800/367-9663 A distributor of preprimed siding	Maibec Industries 660 Rue Lenoir Sainte-Foy, Quebec G1X 3W3 Canada 800/363-1930 Preprimed wood siding
Cape Cod Siding 1948 Hammonds Plains Rd. Bedford, NS B4A 1A0 Canada 800/565-7577 www.capcod.ca Preprimed lodgepole pine siding	Olympic Factory Finish Group P.O. Box 6 Accord, MA 02018 800/266-5143 Manufacturer of primer, stains, and paints for machine finishers
Churchill Coatings P.O. Box 2222 Grafton, MA 01519 508/839-9700 Factory finisher	Skookum Lumber Co. P.O. Box 7309 Olympia, WA 98507-7309 360/426-3944 Preprimed cedar siding
Goodfellow Inc. 225, Rue Goodfellow Delson, QC B4A 1A0 Canada 450/635-6511 www.gdfellow.com Preprimed and prefinished pine and cedar siding	Windsor Mill P.O. Box 39 Windsor, CA 95492 707/838-7101 Preprimed pine and Douglas fir siding