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InspectAPedia ® Home EXTERIORS of buildings ADHESIVES, EXTERIOR CONSTRUCTION AGE of a BUILDING - how to determine ALGAE, FUNGUS, LICHENS, MOSS ANIMAL ENTRY POINTS in buildings ANIMAL ODORS IN buildings ARCHITECTURE & BUILDING COMPONENT ID ATTIC CONDENSATION CAUSE & CURE BARK SIDE UP on DECKS & STEPS BASEMENT WATERPROOFING BEST CONSTRUCTION PRACTICES GUIDE BRICK FOUNDATIONS & WALLS BRICK STRUCTURAL WALL Loose Bulged BRICK WALL THERMAL EXPANSION CRACKS BRICK VENEER WALL Loose, Bulged BRICK WALL DRAINAGE WEEP HOLES BOOKSTORE - EXTERIORS BUILDING SAFETY HAZARDS GUIDE BUILDING SETTLEMENT CARPENTER ANTS CARPENTER BEES CATCH BASINS CAULK GUN TYPES, CHOICES CAULKS & SEALANTS, EXTERIOR CHIMNEY INSPECTION DIAGNOSIS REPAIR COLUMNS & POSTS, DEFECTS CONTAINER HOUSING CONCRETE FOUNDATIONS CONNECTORS, FASTENERS, TIES CRAWL SPACES DECK & PORCH CONSTRUCTION DECK CONSTRUCTION BEST PRACTICES DECK COLLAPSE Case Study DECK FINISHES COATINGS PRESERVATIVES DECK FLASHING LEAKS, ROT Case Study DEFINITIONS of ENGINEERED WOOD OSB LVL etc DRYWELLS, FRENCH DRAINS for FLAT SITES EARTHQUAKE DAMAGED FOUNDATIONS EIFS & STUCCO EXTERIORS ENVIRONMENTAL HAZARDS - 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Exterior building trim guide to product choices, installation methods, details: this article explains best construction practices in the choice, installation, painting, and maintenance of exterior trim on buildings. We discuss solid wood trim, finger-jointed wood trim, hardboard trim, LVL trim, fiber cement building trim, polymer trim products, and cellular PVC building trim products and details. Green links show where you are. © Copyright 2013 InspectAPedia.com, All Rights Reserved. Author Daniel Friedman. Building Exterior Trim: Product Choices, Installation Details
This article series includes excerpts or adaptations from Best Practices Guide to Residential Construction, by Steven Bliss, courtesy of Wiley & Sons. But the original sketch (at left) showing how to place wood boards & trim when exposed to weather outdoors is not correct - details are at BARK SIDE UP on DECKS & STEPS Also see EXTERIORS of buildings. For trim selection and installation at building interiors, see TRIM, INTERIOR INSTALLATION. Our photograph (above) is of the Justin Morrill Smith Homestead, VT, 1840 - more examples of architectural styles including building trim styles can be found at Building Age & Architectural Style. Wood and Composite Exterior Trim Choices for Building ExteriorsAs costs rise and quality levels fall for solid wood board stock, builders have become more receptive to a wide range of alternative products introduced over the past 10 to 15 years. Some of the products are variations on the material used in hardboard siding, a product that has been largely discontinued due to widespread problems with moisture absorption and buckling. Others are fiber cement- based and offer the same durability and longevity as the siding. Still others make use of PVC, urethane, or
other types of plastics, which promise longevity and low
maintenance but may cost significantly more than the solid
wood they replace (see Table 1-10 below).
Guide to Solid Wood Trim on buildingsSolid wood is still the first choice of many builders for highly visible trim such as porch columns that require tight miters and smooth edges and need to tolerate a certain amount of wear and tear. Softwoods have served well in this capacity for many years, since they were traditionally inexpensive, dimensionally stable, and held paint well. Decay Resistance of Modern Wood Trim As smaller, faster-growing trees replace older virgin timber stands, high-quality wood has become more expensive and harder to find. Even when using decay-resistant species, the smaller trees harvested today have less heartwood, which is where the extractives are found in sufficient quantities to be effective against decay (Table 1-11). With any wood species, the sapwood is more prone to decay. Paintability of Wood TrimSolid wood has virtually no shrinkage along the grain and, if finished on all sides, limited seasonal movement across the grain. In general, the denser a wood species is, the more it shrinks and swells with changes in moisture and the worse it is as a substrate for paint on a building’s exterior (Table 1-12).
Vertical-grain, or “edge-grain,” softwoods, such as vertical-grain cedar or redwood, are the most stable and hold paint the best. The flat-grained woods more commonly used as trim are more prone to cupping and other moisture movement and do not hold paint as well.
Finger-Jointed or "Finger Spliced" Wood Used as Exterior Building TrimMany manufacturers now offer solid wood trim made up of short lengths of high-quality lumber that is finger jointed and, in some cases, edge-glued to make boards as long as 24 feet. As with solid lumber, finger-jointed lumber is available in a number of grades and species. Telegraphing at Finger-Jointed Wood Trim JointsIn general, finger-jointed stock is durable and dimensionally stable since short pieces of wood are less likely to warp and twist. The main concern is whether the glue joints will telegraph through the paint as the material swells and shrinks in response to changes in relative humidity.
Because no two pieces of wood swell and shrink at exactly the same rate, the joints often do show through. The best protection is to keep excess moisture out of the wood by starting with quality pre primed stock or using a high-quality water-resistant prime coat and two top coats of paint. When purchasing finger-jointed trim, look for long term warranties against any delamination or glue lines telegraphing through the paint. As with any wood-based product, minimizing exposure to water and maintaining the finish are important for long-term performance. Use of finger-jointed trim boards in building interiors is discussed separately and includes additional photos of finger jointed wood at TRIM, INTERIOR INSTALLATION. Performance of Hardboard Trim on Building Exteriors
Hardboard consists primarily of ground wood fibers and phenolic resin, the same adhesive used in exterior-grade plywood, along with additives to improve weather resistance. Hardboard trim is typically available with either a smooth finish or a wood-grain texture and is sold in 16-foot lengths. Hardboard weighs about 4 pounds per square foot for 1-inch stock, roughly twice as much as softwood. Our photo of badly deteriorated hardboard siding, representative of failures with this type of product, is provided courtesy of Carson Dunlop Associates. Paintability and Dimensional Stability of Hardboard Exterior TrimTo its credit, hardboard trim is very uniform in consistency and holds paint well. However, because it has no grain, it shrinks and swells equally in all dimensions—up to twice as much as wood along its length. In very dry or very humid climates this can lead to gaps or buckling over long sections. Most manufacturers recommend leaving an 1/8-inch gap at butt joints and caulking (CAULKS & SEALANTS, EXTERIOR) with a high-quality paintable sealant to avoid problems. Workability of Hardboard Exterior TrimWhile hardboard is relatively easy to nail, it does not hold nails well and is prone to split if edge nailed. Drilling pilot holes will help. Compared to solid wood, it is more difficult to set nails and fill the holes in hardboard. Finish nails and pneumatic nails tend to pucker the surface, which must be sanded smooth before filling the holes. Round-headed nails driven straight in at a 90° angle leave a clean hole for filling. Most manufacturers recommend face nailing flush to the surface of the board to avoid these problems. If nails do penetrate the surface, sand the material smooth and fill the hole before painting. Water Penetration Concerns with Hardboard Building Trim
OPINION-DJF: details about what to expect when using hardboard trim on buildings are at the following articles: SIDING HARDBOARD Laminated-Veneer Lumber (LVL) Trim on Building Exteriors
Like LVL beams, LVL trim is dimensionally stable and is easy to cut, nail, and install, similar to a piece of plywood. Its weight falls in between solid wood and hardboard. It can be used for fascia, casings, corner boards, and most other exterior trim, and is available in lengths from 8 to 24 feet. According to Laminated Veneer Lumber, Overview [Quoting:]
The example photograph of LVL boards used to form board-and-batten trim on a building exterior is from the above document. LVL Trim Facing MaterialsOne manufacturer, Pacific Wood Laminates, makes a pre primed Douglas-fir LVL trim faced either with textured wood veneer (Socomi Lam®) or medium-density overlay (Clear Lam®). Medium-density overlay (MDO) is a highly durable resin-impregnated paper that resists surface checking and holds paint well. It has a 20-year track record as a durable facing in concrete forms, outdoor signs, and other exterior applications. Georgia Pacific Wood’s LVL trim is sealed on all edges with a water-based elastomeric coating that hides the end grain and resists moisture penetration. It is then primed on all faces, with a second prime coat applied to the finish face and edges. Exterior LVL Trim InstallationAs with other engineered trim products, all cut ends of LVL should be primed in the field and butt joints gapped 1/8-inch and caulked (CAULKS & SEALANTS, EXTERIOR). Miters and scarf joints can be used. Cut ends should be kept 6 inches off the ground, concrete, or other wet materials. Resistance to swelling and delaminating will be similar to plywood siding panels such as T-111. The manufacturer recommends that the nails be set below the surface either by hand or pneumatic nailer and caulked. Use a small-headed finish nail or siding nail so as not to make too large a hole in the MDO facing. LVL is defined and explained at DEFINITIONS of ENGINEERED WOOD OSB LVL etc MDO Plywood Facing for SoffitsConventional plywood with MDO facing also makes an attractive and economical smooth soffit material. Combined with LVL fascia, this should produce a durable and attractive eaves detail. "MDO" plywood is constructed of a core of overlapping veneers of wood, but with a surface layer of medium density fiber. The resulting product has a smooth surface. MDO plywood can be used once as a concrete form material, but should not be re-used for that purpose. MDO plywood is further defined and explained at DEFINITIONS of ENGINEERED WOOD OSB LVL etc APA (The Engineered Wood Association) provides an HDO/MDO Plywood Product Guide that offers details about these products. Fiber-Cement Trim Used on Building ExteriorsContractors who install fiber-cement siding are looking for equally durable materials to trim out their houses. Some have found their answer in fiber-cement trim, which boasts the same dimensional stability, paint holding ability, and resistance to rot, insects, and fire as the siding. Standard 1 7/16 -inch fiber-cement trim is comparable in price to mid grade softwoods and is available with either a smooth face or a wood-grain texture. Workability of Fiber Cement Exterior Building TrimLike the siding, fiber-cement trim is heavy, requires special tools and procedures to cut and machine, and presents a dust hazard to those cutting the material. Although special diamond-tipped tools make it easier to cut and form, standard fiber-cement trim is not easy to miter or to make complex cuts in, and it is too hard to hand-nail. Since it is not a nail base, all pieces need solid wood backing. Like the siding, fiber-cement trim must be face-nailed with the nails set flush to the surface. Applications of Fiber Cement Exterior Building Trim - SoffitsBecause of its durability and resistance to moisture, fiber-cement is a good choice for soffits, using 1/4 -inch-thick panels. For other trim, most manufacturers sell 1 7/16-inch planks, which contractors typically build out with 1/2 -inch plywood or oriented-strand board (OSB) to create a thicker profile. In some markets 3/4 -inch fiber cement- board stock is also available, eliminating the need to fur out the trim but adding considerable weight. Also see SIDING, FIBER CEMENT. Lightweight Option Low-Density Fiber-Cement Trim Boards (James Hardie)To simplify installation, James Hardie Building Products has introduced a low-density fiber-cement trim board called HardiTrim XLD that handles and installs more like wood trim. The new material can be installed with pneumatic finish nails set below the surface and puttied. Unlike standard fiber-cement, XLD holds nails and can be mitered and edge-nailed, simplifying details like corner boards. The 1-inch-thick material weighs about 4 1/2 pounds per square foot versus about 2 pounds for an equal sized piece of white pine. Also see SIDING HARDBOARD and see JamesHardie HardiePlank Siding. Polymer Moldings on Building Exteriors: Dentils, Window Pediments, Decorative Brackets
Installation of Polymer Molding Trim on buildingsMost material is sold preprimed and can be cut, planed, and sanded like wood, only more easily because of the lighter weight. Polymer moldings are installed with adhesive rather than nails, although a few finish nails are often used to hold it in place while the glue dries. Polymer molding trim butt joints and miters are bonded with the same adhesive used to hold the molding to walls or soffits. Most manufacturers provide a proprietary adhesive for installation of polymer molding trim and building details. Because polyurethane foam expands and contracts with changes in temperature, installers should cut long runs of molding 1/8 to 1/4-inch long and “spring-fit” the material into place. If installed slightly compressed, the molding will not leave gaps when the temperature drops and the material shrinks. When joining two pieces of polymer molding outdoors, apply a generous amount of adhesive on both surfaces and clean the squeeze-out with a putty knife. Solvent may be needed to clean adhesive from the joint after it dries. Painting Polymer Molding Trim on buildingsBefore painting, fill any holes or dents with an exterior spackling compound and paint. Avoid leaving cut edges exposed, since without its hard skin, the material has a rough, irregular surface, even after painting. Since polyurethane will degrade from prolonged exposure to UV radiation, it should be painted soon after installation. If the surface is undamaged, it holds paint well. Cellular PVC Moldings - Plastic or Cellular Vinyl Building Trim InstallationPlastic or Cellular Vinyl Building Trim Boards, Panels, and MoldingsTrim stock is available in thickness from 5/8 to 1 inch and in lengths up to 20 feet. Sheet stock, which can be used for soffits and other panel applications, is available in 3/8- to 1-inch thicknesses, with a smooth or beaded face. Manufacturer Marley Moldings makes a wide array of molding profiles, while Azek offers a wide range of trim materials, including a 1-inchthick prefabricated corner board for use with any type of siding (Figure 1-34 above.) Our photos (below, from a Home Depot® building supply store) show examples of Veranda™ Cellular Vinyl mouldings that the manufacturer describes as "Specially formulated to withstand termites, moisture, and rot."
Workability of Plastic or Cellular Vinyl Building TrimSimilar in density to pine, expanded PVC can be cut, drilled, sanded, and even routed like wood. It has moderately good nail holding ability and is installed similarly to wood trim, although with allowances for thermal expansion and contraction. Allow for Thermal Expansion of Plastic or Cellular Vinyl Building TrimAlthough it has less thermal expansion than polymer (polyurethane) foam, cellular PVC expands and contracts considerably more than wood, so it requires special detailing to avoid gaps, buckling, and other movement problems. Manufacturers recommend leaving a 1/8-inch gap for movement for every 18 feet of length. This space is required where trim terminates into an inside corner or against an intersecting piece of trim, such as where corner boards meet the frieze board. A small back cut at the end of the board can help conceal the gap. This amount of movement assumes that the PVC trim board is secured with two nails every 16 inches on-center to a solid substrate (three nails for boards 12 inches or wider). Nailing along the length of the board restricts its overall movement. You can further restrict movement in PVC trim by gluing it to a wood substrate with construction adhesive. Installation of Plastic or Cellular Vinyl Building Trim
Using PVC cement to weld one piece to the next, contractors can fashion corner boards, window surrounds, or other trim assemblies, using screws or nails to hold the pieces in position while the glue dries (Figure 1-35). The assembled sections are then nailed or glued in place. For long runs like fascia, scarf joints are best since they provide the greatest gluing area. Manufacturers recommend nailing on each side of the scarf joint to hold the trim in place while the glue dries. Construction adhesive behind the scarf joint—for example, between the PVC fascia and the wood subfascia—will also help reinforce the joint. As with other PVC joints, the plastic surfaces must be in direct contact to bond properly since the glue will not fill any voids. Predrilling is generally not necessary as long as you use small diameter, blunt-head nails. Either galvanized or stainless-steel box nails are recommended, but contractors have used pneumatic nailers with ring-shank nails successfully. In general, the nailing pattern is the same as for wood trim except nails should be kept 2 inches from the ends of boards. Typically nails are set and puttied before painting. Long Runs of Plastic or Cellular Vinyl Building Trim on FasciasLong runs of fascia require expansion space at each end or in the middle. Where a long run turns a corner, for example on a hip roof, one option is to glue the outside corners and leave an 1/ 8-inch space along the center of each run where it will be less conspicuous than at the corner. To hide the joint, either leave an unglued scarf joint or butt the two pieces of fascia and fill the gap with a polyurethane caulk (CAULKS & SEALANTS, EXTERIOR). If building in hot weather, it is best to construct tight joints, since they will open in colder weather as the material contracts. As Jim Blahut comments, special methods are needed to avoid open butt joints on long trim sections, including gluing, back gluing, 45 degree bevel cuts, and extra nails at the butt joints, and in very cold weather installations, Blahut staggers nails 3/4" away from board edges to avoid cracking :
Painting Plastic or Cellular Vinyl Building Trim - Optional for Color ChangeLike PVC windows and vinyl siding, PVC trim has UV inhibitors and does not need painting. However, if you want a color other than white, use 100% acrylic latex paint and avoid dark colors, which can cause the vinyl to overheat in direct sunlight. PVC trim requires no special preparation to paint and reportedly holds paint well. Mark Morsching, Everflashing, adds: "I use Azek® Trim boards and they say that if you use 100% acrylic exterior house paint it bonds muscularly in about 30 days." Azek indicates that AZEK Trim™ does not require painting but that it can be painted with dark colors using Sherwin-Williams VinylSafe™ paints, Pittsburgh Paints - PPG Architectural Coatings, Duron Paints, ICI Paints, on AZEK trim products. Questions and Answers About Vinyl & PVC Trim & MoldingsQuestion: What type of glue is used to lap or butt-joint vinyl molding?What type of glue is used to glue(weld) lap/ butt joint using this vinyl molding? Thank you for any help you can offer. - R. Whitney Reply - courtesy of Steven Bliss If you’re talking about expanded PVC products like Azek, these vinyl or PVC trim and molding products are fastened with PVC cement. In addition, special detailing is required at PVC trim and moulding joints, and allowances must be made for expansion and contraction. Details about PVC trim installation are in this article at Cellular PVC Moldings - Plastic or Cellular Vinyl Building Trim Installation. Check the Azek website for details or see this vinyl trim AZEK installation guide: www.azek.com/pics/installation_document/AZEK-Trim-and-Moulding-Complete-Installation-Guidelines-2011.pdf Also see "PVC trim holds paint well, resists rot, and looks just like wood", Jim Blahut, The Journal of Light Construction, September 2002. [copy on file]. Exterior Building Trim Installation Details
Caulking trim joints with sealants (CAULKS & SEALANTS, EXTERIOR) is a double edged sword, since all caulk joints will eventually fail, and when they do, the remaining sealant will tend to keep the joint from drying.
Regardless of the specific detail, the following principles will help create long-lasting exterior trim on any building:
-- Adapted with permission from Best Practices Guide to Residential Construction. Resources for Building Exterior ProductsFlashing Tapes and MembranesAvenco www.avenco.com Butyl flashing tape Bakor, Inc. www.bakor.com Blueskin self-adhesive, rubberized-asphalt flashing tape Carlisle Coatings and Waterproofing www.carlisle-ccw.com Self-adhesive, rubberized-asphalt flashing tapes and membranes Dupont www.construction.tyvek.com StraightFlash and moldable FlexWrap Butyl flashing tapes Fortifiber www.fortifiber.com Moistop and FortiFlash self-adhesive and nonstick rubberized-asphalt flashing tape Grace Construction Products www.graceconstruction.com Vycor self-adhesive, rubberized-asphalt flashing tapes and membranes Illbruck Sealant Systems www.willseal.com/usa Self-adhesive butyl and foil-faced butyl flashing tapes MFM Building Products Corp. www.mfmbp.com FlexWrap (foil-faced) and FutureFlash self-adhesive, rubberized-asphalt flashing tapes and membranes Polyguard Products www.polyguardproducts.com Windowseal self-adhesive, rubberized-asphalt flashing tapes and membranes Protecto Wrap Co. www.protectowrap.com Standard and moldable (Protecto Flex) self-adhesive, rubberized-asphalt flashing tapes Sandell Manufacturing Co. www.sandellmfg.com Rubberized-asphalt, PVC, and EPDM flashing tapes Fiber-Cement Siding and Trim Suppliers
Exterior Insulation and Finish Systems (EIFS)Dryvit Systems www.dryvit.com Parex www.parex.com Senergy www.senergyeifs.com Sto Corp. www.stocorp.com Hardboard TrimABTco www.abtco.com The Collins Companies www.collinswood.com Georgia-Pacific www.gp.com Masonite Corp. www.masonite.co Temple-Inland Forest Products www.templeinland.com Polyurethane TrimCustom Decorative Mouldings (CDM) www.custom-moulding.com Focal Point Architectural Products www.focalpointap.com Flex Trim www.flextrim.com Flexible polymer composite moldings 48 CHAPTER 1 | Exterior Finish c01.qxd 10/10/05 14:49 Page 48 Fypon www.fypon.com Mid-America Building Products www.midamericabuilding.com Nu-Wood Decorative Millwork www.nu-wood.com Outwater Plastics Industries, Inc. www.outwater.com Ras Industries www.rasindustries.com Resin Art www.resinart.com Duraflex flexible moldings Cellular Polyvinyl Chloride (PVC) Trim AZEK Trimboards www.azek.com Edge Building Products www.permatrimboard.com Gossen Corp. www.gossencorp.com Marley Moldings www.marleymoldings.com LVL TrimPacific Wood Laminates www.pwlonline.com Clear Lam (textured wood facing) and Socomi Lam (MDO facing) Caulks and SealantsBostik www.bostikfindley.com Construction Sealant (Caulks) Suppliers & ProductsChemrex www.chemrex.com Polyurethanes and other high-performance sealantsDAP www.dap.com Acrylic latex caulks Dow Corning Sealants www.dowcorningsealants.com Silicone sealants GE Silicones www.gesilicones.com Silicone sealant Geocel Corp www.geocelusa.com Acrylic latex, tripolymer, copolymer, Kraton, and clear sealants Macklanburg-Duncan www.mdteam.com Acrylic latex sealants OSI Sealants Inc www.osisealants.com Polyseamseal PVA-based caulk. Pro Series includes latex, polyurethane, and Kraton sealants. www.phenoseal.com Phenoseal vinyl adhesive caulk Red Devil www.reddevil.com Acrylic, silicone, and butyl sealants Sashco Sealants www.sashco.com Big Stretch and Mor-Flexx water-based sealants, Lexel Kraton sealant Sika Corp www.sikaconstruction.com Complete line of Sikaflex polyurethane-based sealants, butyl sealant Tremco Inc. www.tremcosealants.com High-performance, architectural-grade sealants, UGL www.ugl.com Acrylic latex caulks White Lightning www.wlcaulk.com Tripolymer, butyl, polyurethane, silicone, elastomeric, and other high-performance sealants Exterior AdhesivesAbatron EpoxyWood Repair/Restoration 5501 - 95th Avenue, Kenosha, WI 53144 USA Tel: (262) 653-2000
Fax: (262) 653-2019, For orders: (800) 445-1754 Abatron provides a wide range of wood & building restoration epoxy products including a low-viscosity system that penetrates and solidifies rotted wood and a thicker wood repair putty type epoxy. Ambel www.excelglue.com Excel polyurethane glue Elmer’s Products www.elmers.com Yellow glues, ProBond polyurethane glue Custom-Pak Adhesives www.custompak.com Resorcinol and yellow glues DAP Inc. www.dap.com Weldwood contact cement, resorcinol, and construction adhesives Resources 49 Franklin International www.titebond.com Titebond yellow glue, Liquid Hide Glue, and construction adhesives Gloucester Co. Inc. www.phenoseal.com Phenoseal adhesive caulk Gougeon Brothers www.westsystem.com West System epoxy Gorilla Group www.gorillaglue.com Gorilla polyurethane glue MACCO Adhesives www.liquidnails.com Liquid Nails construction adhesive SI Sealants www.osisealants.com PL400 construction adhesive System Three Resins www.systemthree.com Quick Cure epoxy Water-Repellent Preservatives (WRPs)Cuprinol www.cuprinol.com Cuprinol Clear Wood Preservative Dap www.dap.com DAP Woodlife Wolman www.wolman.com Premium Water-Repellent Sealer For More Information on Building Practices for Exterior Wall ProductsCalifornia Redwood Association www.calredwood.org Cedar Shake and Shingle Bureau www.cedarbureau.org USDA Forest Products Laboratory (FPL) www.fpl.fs.fed.us Vinyl Siding Institute www.vinylsiding.org Western Wood Products Association (WWPA) www.wwpa.org -- Adapted with permission from Best Practices Guide to Residential Construction. Frequently Asked Questions (FAQs)... Ask a Question or Search InspectApediaQuestions & answers or comments about how to choose, install, maintain or repair exterior building trim. Ask a Question or Enter Search Terms in the InspectApedia search box just below. Technical Reviewers & ReferencesRelated Topics, found near the top of this page suggest articles closely related to this one.
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