POST UPDATED 12May2017: Exterior adhesives for gluing wood (and other things) are different than what makes sense for indoor applications. The challenge is 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. And while the 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.
Exterior Adhesive: Cross-linking PVA Glue
Of these options, cross-linking PVA is the cheapest and easiest exterior adhesive to use. I like them best for general-purpose outdoor gluing, though these are not waterproof glues. 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 to 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. 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 is Franklin’s Titebond III. It’s really something.
Cross-Linking PVA Pros:
- 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
- No usually suitable for submerged applications
- More expensive than ordinary PVA
- Outdoor furniture and wooden architectural details
- Indoor woodwork that could be exposed to water accidentally
Exterior Adhesive: Polyurethane Adhesives
Polyurethane glue is waterproof enough to use in all applications above the waterline, also including situations where one or both surfaces are made of plastic or metal. 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. 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.
Polyurethane Glue Pros:
- Bonds non-porous and porous materials
- Needs no air to cure
Polyurethane Glue Cons:
- Requires small amounts of moisture to cure
- Potentially messy
- Hardens in bottle
- Outdoor woodworking projects, especially those bonding wood with metal or plastic
Exterior Adhesive: Epoxy Glue
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 positioned and clamped for up to 12 hours after mixing.
- Some formulations are rated for use below water line
- Forms very strong bond
- Cures in a wide variety of conditions
- Bonds porous and non-porous materials
- Expensive – 12 ounces costs $20 to $30
- Difficult clean up
- Requires mixing of two parts
- Marine applications
- Gluing jobs in cold and wet conditions
Exterior Adhesive: 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 application. The subfloor you see to the right 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
- 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.
Exterior Glue Q&A:
A: I can’t think of any way to reliably fasten wood veneer to a garage door. It would be difficult to install neatly, but more importantly the veneer wouldn’t last long out in the elements. Even if you kept it varnished, I think it would deteriorate. There are exterior glues out there that would do the job, but it’s the wood that’s the limiting factor. If a wood look is what you’re interested in, then perhaps you could get someone to paint on some faux wood patterns. These look pretty good from a distance. It’s an old technique that was very popular a hundred years ago. Sorry I couldn’t be more positive about your idea.
Gluing Construction Lumber
Q: What kind of glue should I use to glue exterior lumber? I’m planning to glue two 2x6s together to make 3″x5 1/2″ tongue&groove lumber for cabin walls. I tried Titebond II, but it’s too thin.
A: One option that will work very well is a construction adhesive called PL Premium. It comes in caulking tubes, is completely waterproof and quite viscous. Don’t use other construction adhesives. PL Premium is far superior. I’ve never seen anything else work as well. All this said, you might consider finding a small sawmill somewhere to mill some 3×6 for you. There are mills like this all over the country. The cost of custom-milled wood will similar to your plan, and you won’t have the trouble of lamination. Just a thought.
Rebuilding an RV Floor
Q: How should I make a replacement for the slide-out floor in my motor home? The wood has rotted and I’m thinking of gluing several layers of plywood together to make a new floor. What glue should I use?
A: Your plan is good. Make sure you use an exterior grade plywood (pressure treated is even better) and make your total floor thickness at least as thick as the current floor. These days there are more water resistant glues than were available in the past, and one of my favourites is called Titebond III. It comes in a bottle and looks like regular wood glue, but it’s more than water resistant enough for your application. I’d do the lamination work on a large, flat concrete floor, then use a roller to apply the glue to both mating surfaces of each piece of plywood. These sheets are going to want to slide on each other when they come together, so start by using small finishing nails to hold things aligned – one in each corner. I wouldn’t use screws to pull the sheets together, but rather weights resting on pieces of lumber to spread the pressure.
Gluing Outdoor Tiles
Q: What should I use to glue ceramic tiles to a pressure treated planter? I live in Minnesota, so it gets hot in the summer and really cold in winter. How can I accomplish this with the weather conditions of moisture and temperature changes?