Exterior glues & adhesives:
This article series discusses best practices construction details for building exteriors, including water and air barriers, building flashing products & installation, wood siding material choices & installation, vinyl siding, stucco exteriors, building trim, exterior caulks and sealants, exterior building adhesives, and choices and application of exterior finishes on buildings: paints, stains.
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Portions of this article were adapted/paraphrased and used with permission from Best Practices Guide to Residential Construction by Steven Bliss.
Watch out: No exterior millwork should rely entirely on adhesives,
since no glue is 100% waterproof, and any adhesive can
fail with enough moisture cycling and movement in the
wood. It is always wise to back up an exterior glue joint
with mechanical fasteners, design the woodwork to shed
water, and protect it with a good paint job. Still, there are
several good options for gluing exterior work that should
last indefinitely if well maintained (see Table 1-14) given just below. Also
[Click to enlarge any image].
There are several factors to consider in selecting a glue. For exterior woodwork, the biggest concerns are typically water resistance, strength, and cleanup. Working temperatures, clamping time, and gap filling abilities may also be important, depending on the specific job and conditions.
A glue’s water resistance is classified as Type I or Type II. A Type I designation indicates that the glue bond can survive repeated submerging in boiling water. Type 1 glues are used for laminating structural timbers such as glulams. The most common Type 1 glue, resorcinol, has strict temperature and clamping requirements and is rarely used on residential job sites. Type II glues must maintain their bond after being soaked for four hours and then dried three successive times. These are suitable for all but the most punishing residential applications.
One-part polyurethane glues have grown in popularity over the past few years due to their excellent strength and increased water resistance compared to yellow glue. Since polyurethane requires moisture to cure, it will bond to wood that has up to 25% moisture content.
For wood that has less than 10% moisture content or appears dry, you should moisten one of the two surfaces being joined with a sprayer or damp cloth and apply a thin coating of glue to the other. Polyurethane bonds well to wood, stone, most metals (not stainless steel), and ceramics, as long as at least one of the surfaces being glued is porous.
Polyurethane foams up as it cures, expanding to three to four times its original size and filling any small gaps in the joint. But unlike epoxy, the filled gaps have no strength. Clamping time is one to four hours, depending on the specific formulation. For maximum strength, clamp for 24 hours.
Because of its tenacious grip, you should protect any materials or finished surfaces from drips and protect your hands with latex gloves, as the glue cannot be removed except by abrasive cleansers.
If wet glue drips onto a finished surface, wipe with a dry cloth, since anything wet will activate curing. After the glue has dried, scrape away the squeeze-out with a sharp chisel and sand any residue. The glue dries to a brownish tan, which can be painted.
OPINION-DJF: we really enjoy almost everything about Gorilla Glue (photo above), and have used it very extensively in both repair (it will expand and secure a wobbly chair leg) and in construction indoors and out. Our favorite indoor project using this glue was building site-built built-up custom curved trim. Our photos (below) show the an in-process stage of constructing custom wood trim in a home as well as the finished result after sanding and finishing.
We ripped and planed a mix of Brazilian cherry and white oak into narrow strips that were bent to shape, glued, and clamped in place. Critical was to use enough wedges and later clamps to be sure that our wood strips remained closely aligned with no open gaps during drying. We cut away the flue overflow after it dried, by hand, using a sharp chisel. A final sanding with an orbit sander and a router to round the edge of our curved trim/stair was all that it took to produce a beautiful end product.
What's not to enjoy: working wearing protective plastic gloves proved impossible - as soon as the gloves got sticky with glue it was impossible to pick up anything. Working bare-handed means you will have glue on your hands for several days. Washing the dishes in the kitchen sink every day will speed wearing off of this ooky mess.
Continuing from from Best Practices Guide to Residential Construction:
Long the adhesive of choice for boat builders, epoxy has high adhesive strength and rigidity, low shrinkage, and good resistance to water and chemicals. It bonds well to wood, concrete, foam insulation, and other porous substrates—and to nonporous surfaces as long as they are lightly roughed up.
epoxy is comparable in strength and water resistance to polyurethanes, requires minimal clamping, and can fill gaps with little loss of strength, making it an ideal choice for less-than-perfect carpentry joints.
In our photo (left) an example of Loctite™ Epoxy is on the right.
As a two-part system—with various hardeners to choose from and additives such as fillers to improve gap filling—epoxy is also the most complicated and costly approach. Once the resin and hardener are mixed, the working time ranges from about 10 minutes to an hour, depending on the ambient temperature and whether a slow or fast hardener is used. Heat speeds up the curing, so a slow hardener is recommended in hot weather, a fast hardener in cold weather. It is important to mix the correct proportions and mix thoroughly or un reacted resin or hardener may remain in the cured epoxy.
For best results, use a disposable brush to coat both sides of the joint with liquid epoxy. After coating the joint, add fillers to the mix if required. Fillers change the viscosity of the mix and enable it to bridge gaps with minimal loss of strength (you can bridge small gaps up to about 1/16-inch without fillers).
A small amount of filler helps keep the mix from running. Once the fillers have been added, apply the thickened epoxy to one side of the joint and clamp just enough to squeeze out a little epoxy. A common mistake with epoxy is clamping too tightly. This will create a weak, “glue-starved” joint. Cleanup of the wet epoxy requires solvents such as acetone or lacquer thinner. Workers should use rubber gloves to protect their skin.
Any kind of clamping that holds the joint still is suitable, including staples, nails, or wood screws. Scrape off any squeeze-out with a putty knife or dry rag. Once the epoxy has cured to a solid state that cannot be dented with a fingernail, it has reached 90% of its final strength. Then the clamps can be removed and any excess sanded off. The epoxy continues to gain strength for several days and is paintable.
Where clamping is not practical, another option is hot melt polyurethane. Hot-melt polyurethanes have been used in industrial settings for many years, but they have only recently been introduced for job-site use.
Unlike its moisture-cured cousin, hot-melt polyurethane does not foam up and needs no clamping.
It sets in about 30 seconds and provides the same level of water resistance as regular polyurethane but less than half the strength.
Still, this is more than enough for many applications. Remove any squeeze-out with a putty knife or scraper as soon as it firms up, as it is difficult to sand clean.
Hot melt glues are sold in a variety of formulations and colours and can be used on both porous and non-porous surfaces.
Watch out: getting hot melt glue on your fingers is likely to result in an unpleasant burn. Hotmelt glues flow from a low-melting-temperature glue gun at about 250°F and from higher temperature hotmelt glue guns at 380°F.
Since Steve's original chapter on exterior glues and adhesives was penned (ok well types), several of the stunning superglue producers have developed super glue products that perform well in outdoor or "exterior" applications.
Super glues are technically cyanoacrylate adhesives (of the formula Ethyl cyanoacrylate). These glues dry clear, bond very quickly, and are very strong provided that you follow the manufacturer's instructions and provided that the surfaces are suitably smooth. Slow-setting super glues and gel type superglues are adaptations of the original product that can make working with super-glue easier. Super glues are available for bonding a wide range of products, even concrete, steel and wood.
Common names for these instant bonding adhesives inclulde Krazy Glue and Superglue (the original), and pegamento instante.
Watch out: if using a super glue in a construction application choose the proper glue product for the materials your are bonding and also for interior or exterior use. Some cyanoacrylate adhesives super glues, but not all, are suitable for exterior use.
Exterior rated super glues include (this is not a complete list) LePage® Gel Control ® Super Glue, LePage® Ultra Gel Control® Super Glue, Loctite® Super Glue ULTRA Gel Control™ ,
An example of an exterior-rated super-glue is Loctite® Super Glue Ultra Gel Control™ shown at left and described by the company as follows:
Watch out: getting superglue in the eye can result in a serious eye injury "ocular superglue injury" (McLean 1997 and others) as we cite at REFERENCES
Watch out: superglue bonds instantly to the skin (Daniell 1997 and others). Getting super glue on your fingers can result in tearing of skin before you can pry your hands apart. This substance would in special and personal appications be more effective than the famous astringent mother-in-law plant, but for legal reasons (we have no liability insurance) we don't recommend its application on a human being unless that use is for medical purposes and supervised by a physician.
Superglues used in Medical Applications are Different
Superglue in a somewhat different molecular structure (Dermabond Advanced™ [2-Octyl cyanoacrylate] or Histoacryl® Topical Skin Adhesive Butyl cyanoacrylate]) has indeed been used in wound closure and in reconstructive surgery by orthopedic surgeons.
Kennedy (1994) warned that superglues used in medical applications are similar but not identical to those used to glue your hat-band back on. Legat (2004) discusses the use of superglue in the dental field. And though manufacturers rightly warn about eye injury from getting superglue in your eye, there are also medical applications using superglue of the Dermabond type in sealing clear corneal cataract sounds (Rittterbrand 2005).
Report of use of superglue to fix Jim's broken spine
Our friend Jim Kelly was shopping when he bent over the meat cooler. He woke up in hospital. A soda delilvery operator had toppled an overloaded hand truck of soda onto Jim, breaking his back and leaving Jim unconscious. Jim's orthopedic surgeon, in an emergency procedure, used super-glue in the course of repairing damage to Jim's spinal column and remarkably, he was up and walking the next day. (Actually all of them were up and walking the next day, but the soda delivery operator was, we heard, walking the sidewalks looking for a new job.)
Watch out: while some super glues have been specially formulated for use on foamed plastics, most don't work on soft or foamed plastics such as insulating board. For gluing foam board a construction adhesive recommended by the insulating board manufacturer will work and is suitable, for example, when gluing foam board insulation to a basement foundation wall. Spray adhesives can also work for affixing foam board insulation to building surfaces.
3M Corporation offers 3M Super Fast Flex Repair, a 20-second set urethane adhesive that "repalces cyanoacrylates". 3M says that their urethane adhesive will not crack or become brittle like cyanoacrylates and that it bonds to virtually anysubstrate with excellent gap filling properties for bonding small parts.
Above: 3M Automix 8115 fast-set urethane adhesive.
Fast setting epoxies also can be an alternative to superglue for some bonding alternatives.
Polyvinyl acetate (PVA) is the most common glue on residential job sites due to its low price, long shelf life, easy cleanup, and overall ease of use. The Type II version provides good water resistance and provides a very strong bond, and it is only slightly more expensive than the regular yellow glue.
In our photo (left) the yellow Titebond™ glue (at left in the photo) is suitable for the applications discussed here. While Elmers™ also makes a yellow Type II glue, do not use the white "school glue" shown in the center of our photograph for construction purposes. White school glue lacks water resistance and strength and won't work outdoors.
Similar to the older style white glue, yellow glue is formulated with a higher solids content to make it less runny and with other additives to make it set up quicker. Clamp time is about one hour. Any squeeze-out is simple to remove with a damp rag. Once the glue dries, however, it will resist paint and stain and needs to be scraped or sanded off.
In general, yellow glue should not be applied in temperatures below 50°F or allowed to freeze before it cures. In freezing weather, store the glue indoors, since a couple of freeze-thaw cycles may ruin the glue. For exterior work subjected to moderate weather exposure, Type II yellow glue is a good option.
Watch out: White Craft Glue, which in some cases such as Elmers' glue products is sold in a similar-looking bottle is white in color, not yellow, and is not suitable for outdoor applications. Because white craft glues use water as the solvent and carrier for the adhesive, these glues are easy to clean-up during use, are low in toxicity, and are often used in crafts and projects in schools and for children.
White glues or "craft glues" are intended for gluing light, porous materials such as cardboard, cloth, and paper and are not suitable for general construction indoors nor outside.
What's the difference between Type I Yellow Glues and Type II Yellow Glues?
Type I yellow glue is rated for interior use only. Type II yellog glues can be used outdoors.
Watch out: Yellow wood glue, discussed just above, may be sold in both interior-use and exterior-use applications. Interior-use yellow glues are also water-based and use adhesives simlar to those used in white craft glue (vinyl acetate polymers) but are stronger and may be suitable for interior use in buildings such as gluing trim. However a Type I yellow glue that is not marked as "for exterior use" should not be used outdoors.
-- Portions of this article were adapted and used with permission from Best Practices Guide to Residential Construction.
Construction Sealant (Caulks) Suppliers & Products
Polyurethanes and other high-performance sealants
-- Adapted with permission from Best Practices Guide to Residential Construction.
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Technical Reviewers & References
Steven Bliss served as editorial director and co-publisher of The Journal of Light Construction for 16 years and previously as building technology editor for Progressive Builder and Solar Age magazines. He worked in the building trades as a carpenter and design/build contractor for more than ten years and holds a masters degree from the Harvard Graduate School of Education. Excerpts from his recent book, Best Practices Guide to Residential Construction, Wiley (November 18, 2005) ISBN-10: 0471648361, ISBN-13: 978-0471648369, appear throughout this website, with permission and courtesy of Wiley & Sons. Best Practices Guide is available from the publisher, J. Wiley & Sons, and also at Amazon.com
Abstract: We present the case of a 39-year-old woman who applied superglue to her fingertip as a treatment for dry skin. She developed full-thickness necrosis of her thumb pad complicated by a secondary superinfection. This necrosis occurred from the degradation of the cyanoacrylate in the superglue compound to formaldehyde, causing local histotoxicity. This injury necessitated a local flap for coverage, which healed uneventfully and without lasting sequelae.