Glue Choice for Oak Exterior Doors
From Gene Wengert, forum technical advisor:
I have not noted any problems with oak and epoxy if the surfaces are properly prepared and the epoxy is properly mixed. However, do keep in mind that epoxy will degrade with exposure to sunlight, so you should use a stabilized epoxy if the joints will be exposed to light.
From the original questioner:
Thanks for the input. My normal construction method is just what you both have suggested - West Systems with Microfibers. I've never used oak for doors (normally African mahogany) so I was initially worried doing my research on a proper adhesive. The oak doors will be exposed to the sunlight (in Hawaii) but no rain, etc. The glue lines will be tight and the strength comes from the large loose tenons in a 2 1/4" thick frame so hopefully there won't be much degradation of strength due to UV exposure. I would be very grateful for any other tips you may have.
From Gene Wengert, forum technical advisor:
Here is an article that I wrote for CabinetMaker magazine, Copyright 2009.
"Working With Epoxy Adhesives"
In many, if not most, cases, the standard wood working adhesives - the white glues, yellow glues and PUR’s will do the job and perform well. However, in a few situations, such as in a repair job when the joint is not perfect and tight fitting (especially miter joints with all the end grain), when joining wood with a non-wood material, or when dealing with repairing decayed wood pieces, we need an adhesive that will bridge a gap, is highly moisture resistant, is quite strong, is easy to apply, can be used on a weaker joint, and/or can be used to make a fillet to increase the joint strength. In these few cases, epoxy adhesives, even though they are fairly expensive compared to other adhesives, are the perfect choice. Yet epoxy is a bit different than the rest of the adhesives, so let’s look at this adhesive more closely so we use it correctly and are satisfied with the results.
Safety. First and foremost, like many modern adhesives, safety is an issue. For some people, skin sensitivity (in the form of a rash), builds up over time when exposed to the un-cured adhesive. Therefore, use good ventilation and always wear disposable, plastic gloves. Although epoxy does not stick to skin and can be removed when the adhesive hardens, never clean your skin with a solvent, as the solvent will likely aid in getting the adhesive deeper into your skin. Having said all this, allergic reactions not common. One real concern is sanding dust from dried adhesive. Use a good breathing mask.
Chemistry. Epoxy adhesive is made from two separate chemicals. Part A, also called the resin, is a diglycidyl ether of bisphenol A. The second constituent, called Part B or the hardener (and is sometimes mis-called a catalyst) is a reactive amine (nitrogen with hydrogen atoms, similar to ammonia). When mixed together, these two form a fully-crosslinked thermoset plastic adhesive.
Mixing. Part A (the resin) and B (the hardener) are mixed together just before the adhesive will be put on the joint, It is critical is mix the exact proper amounts of Parts A & B and then stir them for 15 to 30 seconds to assure complete and thorough mixing. If there is too much resin in the mix or even part of the mix, the result is a weak joint; if there is too much hardener, there will be moisture sensitivity of cured joint. Mixing is usually done by volume (2:1 or 1:1, resin to hardener). Weight can be used, but then it's 43:100 or less. Obviously, follow label directions. Generally speaking, small batches are better than large.
Application. Surfaces must be clean and free of oils, etc. Coat both surfaces of the joint. Then, and this is where most people make a mistake, do not use too much pressure. A good epoxy joint is a thick joint. Oftentimes, we will use screws, nails, or fast-curing epoxy to hold the joint together until the main epoxy adhesive cures.
The rate of cure varies depending on formulation; most woodworkers will use epoxy with a 30 minute curing time, or even longer. The curing rate is also affected by temperature, going twice as fast for every 18 F warmer. Finally, the rate is affected by the surface area. As the epoxy generates its own heat that is needed for curing, if the adhesive is spread thin, then the heat escapes and the adhesive takes longer to cure than a thick spread where the heat is held in.
The mixed adhesive can be made thicker, almost like putty, if wood floor (or other approved fillers) is added. This thick adhesive will not “run” and so can be used to fillet a joint and greatly increase the joint’s strength. This thick adhesive is also useful when bridging a gap. Thicken adhesive would not be used for “normal” gluing applications.
After gluing. The strength of epoxy is only about 60% to 80% in 24 hours (strong enough to stay together), even for the fast cures. It will take 72 hours (or so) to get close to 100%. Avoid high stress on the joint for a few days. To make sanding and finishing easier, scrape off any excess adhesive before it cures. Surface films (called blush) form when the adhesive cures when it is cool and/or humid. These films should be washed off with soap and water on a rag before finishing. Wear a dust mask when sanding.
Special note: Cured epoxy deteriorates rapidly when exposed to uv light. In applications where the joint will be exposed to light, use an opaque paint or varnish with uv inhibitor. Never use polyester finishes on epoxy joints. Avoid finishes that have alkyd listed as an ingredient. Two-part polyurethanes are OK, but not one-part with alkyd resin. Questions? Most reputable adhesive manufacturers can answer additional questions.
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