EXTERIOR ADHESIVES: Choosing the Right Waterproof Outdoor Glue

POST UPDATED 2Nov2017: 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

exterior adhesives PVAOf 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:

  • 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
  • No 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 Adhesive: Polyurethane Adhesives

exterior adhesives polyurethanePolyurethane 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:

  • 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

Uses:

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

Exterior Adhesive: Epoxy Glue

exterior adhesives epoxyEpoxy 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.

Epoxy Pros:

  • 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

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 Adhesive: Construction Glues

exterior adhesivesConstruction 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

Uses:

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

Polyurethane CAULKING: The Non-Glue Glue

exterior adhesive caulkingAlthough 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:

Glue for Outdoor Crafts
Q: What glue is best for making outdoor model houses and fairy gardens? I mostly work with natural materials such as wood, sticks, toothpicks – tiny stuff like that. I’ve been using a small hot melt glue gun and love working with it, but the projects don’t hold together long term. Any suggestions?

A: That sounds like an interesting project. I think you’ll find Titebond III works very well. It looks just like regular wood glue, but it’s very resistant to water. The only challenge might be the need to hold the parts for 30 minutes or so while this glue grabs. Hot melt is more or less instant, but any liquid glue like Titebond III is not. You might consider a strategy of applying the Titebond III to part of the joint, but also using a small blob of hot melt to one side just to hold the parts together while the Titebond III dries. How does this sound? Please let me know how you make out.

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Gluing Wood Veneer to a Door
 Q: How can I fasten wood veneer to my metal garage door? I live in Palm Desert California and the temps get very hot. What is your suggestion?

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.
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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.

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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.

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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?

A: That’s a great question. The best option I can think of is a particular construction adhesive called PL Premium. There are lots of construction adhesives on the market, but this is the only one I know of that’s based on polyurethane. I know from many years of experience that this stuff works very well in all outdoor conditions. It comes in a caulking tube and is nice and thick – perfect for setting tiles. Also, be sure to use tiles rated for outdoor use. Many indoor tiles absorb enough moisture that they will crack and flake as temperatures drop below freezing.
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Gluing a Sign
Q: What do you recommend for bonding an 8″ x 6″ acrylic-backed sign to a tinted exterior glass window? Sign will be outdoors and must have strong adhesion and a clean, clear  installation with no residue.
A: The best approach with unusual situations is to try a test first. Can you get some of the sign material to use as a test on a separate piece of glass? I’d apply spray adhesive to the back of the sign only, then stick the sign to the glass. This will probably work fine, but a test is wise anyway. Another option is the kind of thin, double-sided tape used by artists and layout pros. This is much thinner than double-sided carpet tape, which wouldn’t be suitable. A few strips of the high-end double-sided tape would do a very nice job, though you would see the strips of clear tape from the back. Spray adhesive would give you a uniform look.
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Gluing Vinyl Cladding

Q: How can I re-fasten the vinyl cladding on my patio doors? The vinyl came loose and I removed it completely with a 6″ joint knife so the wood can dry out. The vinyl looks fine and I’d like to re-secure it on the wooden portion of the door. What adhesive would you recommend?  

A: I’ve been considering about your situation and the best option I can think of is contact cement. You need something that’s waterproof, thin enough to allow a tight fit between vinyl and wood, and the ability to bond to both porous and non-porous materials. I’d roll on contact cement on both the wood and the vinyl, let it dry completely, then roll another coat on before letting it dry and applying the vinyl. Just to be sure, I’d also try some experiments using vinyl and scrap wood. I think contact cement will work fine, but you want to be certain of the glue you’re using and your technique.
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Gluing Glass Mosaic Tiles

Q: What weatherproof adhesive should I use to fasten stained glass mosaic tiles to a primed fiberglass sculpture that will be displayed outside?  (See www.conejopublicart.org).  Also, can you recommend a sealant to apply once it’s finished?

A: That sounds like an interesting project. Judging by the website, it looks like the glass tiles would be opaque. Is that right? If it is, then I don’t think you could do better than PL Premium. It’s a very durable, completely weatherproof adhesive that’s also quite thick and gooey. It would hold your mosaic tiles quite well while the adhesive dries. You wouldn’t need any sealer over this, either. Have you ever tried PL Premium? Take care and let me know how you make out. The trick would be learning to apply just enough to do the job without any squeeze out around the tiles.
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Posted on September 27th, 2017

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