EXTERIOR ADHESIVES: Choosing the Right Waterproof Outdoor Glue

Exterior adhesives for gluing wood (and other things) are different than what makes sense for indoor applications. The challenge of keeping things glued together in potentially wet, outdoor conditions is entirely different than working with interior projects that never see moisture, UV rays or wide temperature swings. If you want outdoor projects to endure, you need more than ordinary glue.

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While exterior adhesives are getting better quickly, you’ve still got to understand where each technology works best and why. Wood is the most commonly glued exterior material, and you’ve got four main choices to make it stick. Not all of these are truly waterproof glues, as you’ll see. Cross linking PVA glue (so-called “weatherproof” type I, type II and type III carpenter’s glues), polyurethane glues, epoxies and construction adhesives. Need information about truly waterproof glues? You’ve come to the right place.

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Exterior Adhesives: Cross-linking PVA Glue

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Of all the options, cross-linking PVA is the cheapest and easiest exterior adhesive to use. I like this option best for general-purpose outdoor gluing, and though these glues are not waterproof enough for continuous submersion they work fine for any outdoor application. Cross-linking PVA looks just like regular wood glue, and even cleans up with water before it dries. After hardening, however, cross-linking PVA is more or less moisture-proof because of the transformation of the polymers that occur while drying. Cross-linking PVA won’t hold up indefinitely in submerged situations, but it will keep things together during extended damp weather that would turn ordinary white, yellow or brown carpenter’s glues into mush in a few days. Weatherproof PVA glues are rated by the American National Standards Institute (ANSI), and glues, meeting their “type II” rating offer acceptable performance outdoors. Type I PVA stays hard more reliably after extended exposure to water than regular wood glue. If a PVA glue doesn’t have either of these ratings printed on the bottle, or the word “weatherproof” isn’t mentioned, it’s probably not for use outdoors. These days Type III PVA glues are available and even better. My current favourite for wood is Franklin’s Titebond III. It’s really something. This stuff is strong, it’s easy to work with, easy to clean up and amazingly waterproof when fully dry.

Cross-Linking PVA Pros:

• Inexpensive
• Non-toxic
• No odor
• Can be applied by roller
• Easy clean up when wet

Cross-Linking PVA Cons:

• Some brands go gummy in bottle after 12 to 24 months
• Requires temps above 10ºC while drying for full joint strength
• Not usually suitable for submerged applications
• More expensive than ordinary PVA

Uses:

• Outdoor furniture and wooden architectural details
• Indoor woodwork that could be exposed to water accidentally

Exterior Adhesives: Polyurethane Glue

Polyurethane glue is waterproof enough to use in all exterior adhesive applications above the waterline. The beauty of polyurethane is that it can be used in situations where one or both surfaces are made of plastic or metal, in addition to wood. Unlike PVAs that are best for porous materials, polyurethane glue hardens by chemical reaction with moisture. No air necessary. It’s also easily sandable and takes solvent-based stains reasonably well. On the downside, polyurethane is considerably more expensive than PVA (a 2 ounce bottle of poly costs as much as an 8 ounce bottle of PVA), and polyurethane is messier. You need to remove stray glue with acetone when it’s wet (water won’t work) and polyurethane expands as it cures, sometimes leaving behind large berms of hardened, foamy squeeze out. Polyurethane glue also hardens in the bottle after 6 to 18 months, so the shelf life is fairly short. Gorilla Glue was the first mainstream polyurethane exterior adhesive to hit the market years ago, but to be honest I’ve never noticed that any brand is better than another. I’ve probably used all of them.

Polyurethane Glue Pros:

• Strong
• Bonds non-porous and porous materials
• Needs no air to cure

Polyurethane Glue Cons:

• Expensive
• Requires small amounts of moisture to cure
• Potentially messy
• Hardens in bottle over time

Uses:

• Outdoor woodworking projects, especially those bonding wood with metal or plastic

Exterior Adhesives: Epoxy Glue

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Epoxy is more than just fast-hardening goop in tiny tubes. Industrial-grade epoxies come in various liquid formulations, some for use in unusually cold or wet situations. Some marine-grade versions are rated for continuous exposure below the water line and will even harden under water. Epoxies like these are one of the only truly waterproof glues around, and really go beyond the requirements of exterior adhesives. Downside? Yes, everything has some. Relatively high cost, and messy working regime are a case in point. Two liquids are mixed together, then left to sit undisturbed for a few minutes before the adhesive is spread on parts and clamped up. The long open time of some slow-cure epoxies allow parts to be re-positioned and clamped for up to 12 hours after mixing.

Epoxy Pros:

• Some formulations are rated for use below the water line
• Forms very strong bond
• Cures in a wide variety of conditions
• Bonds porous and non-porous materials

Epoxy Cons:

• Expensive – 12 ounces costs $20 to $30
• Difficult clean up
• Requires mixing of two parts

Uses:

• Marine applications
• Gluing jobs in cold and wet conditions

Exterior Adhesives: Construction Glues

Construction adhesives have done more than anything else to make modern houses solid and squeak-free, but they need to be used properly to perform to their potential. When securing subfloors, for instance, you’ve got to screw down each sheet while the glue is still soft, not after flopping down all the sheets that cover the floor (however more efficient this might be). Also, when conditions are wet on building sites, polyurethane construction adhesives work best. Best by far. Since the chemistry behind these formulations requires small amounts of water to harden, damp joists and plywood are no problem at all. Round up a cordless caulking gun to save your hand muscles and speed up application. The subfloor you see above was glued down with polyurethane construction adhesive in winter. I used a brand called PL Premium. It’s my favourite by far – way better than any other construction adhesive I’ve used.

Construction Adhesive Pros:

• Thick, non-running consistency for vertical and horizontal applications
• Packaged in caulking tubes for rapid application with cordless caulking gun

Construction Adhesive Cons:

• Product stiffens up during cold weather
• Some formulations harden in tubes or damaged by freezing

Uses:

• Subfloor reinforcement
• Application of rigid foam insulation
• General construction applications

Polyurethane CAULKING: The Non-Glue Glue

Although it’s not sold as an adhesive, polyurethane caulking and sealants are some of the best glues you can use to secure non-porous materials in exterior applications. It’s even better than construction adhesives for gluing plastics and sheet metal parts because it’s more squishy, allowing a closer bond between neighbouring components. Here’s a little trick: Polyurethane caulking or sealant is particularly good for joining seams in aluminum fascia. It eliminates the need for most face nails, creating a cleaner appearance with less chance of buckling and waviness from heat expansion. For some reason polyurethane caulking has been getting less common on store shelves since about 2012, but it’s still available and still worth finding. I get mine here on Amazon when I need some.

Exterior Adhesives Q&A:

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Categories: Outdoor