Gazette Archive 4/28/02
The Beginning Woodworkers Friend
Great furniture has been made for many years long before epoxy was invented, so many woodworkers might say, why do we need it. The simple answer is that we don't, but if you are like myself and those joints are not always cut perfectly, epoxy can save you from throwing the wood in the trash and starting over. This article is about how and where to use epoxy as a glue and filler.
The epoxy that I am referring to is a good quality 2-part epoxy - not the cheap 5 minute stuff that most hardware stores sell. 2 well known brands are West System(TM) and System 3(TM). There are other good brands as well. These can be bought at marine supply stores, some home centers, and most woodworking stores. I have personally only worked with West System.
Epoxy consists of a resin and a catalyst which are mixed together in a pre-determined ratio which then harden in an exothermic (heat producing) reaction. One reason that epoxy can be so useful for woodworkers is that as it hardens, there is very little contraction, so it is an excellent product to fill voids that often exist when we make joints that are not quite right. Also, the glue line can be as thick as you want, so loose fitting joints are not a problem. In fact, epoxy must have a thicker glue line than most other glues, so clamping is necessary only if you need to hold the pieces together and the clamping pressure should be as light as you can make it and still hold everything where you need it.
The open time for epoxy varies depending mainly on the temperature and which hardener is used. At 72 degrees, West makes 4 different hardeners which give an open time of 9-50 minutes depending on which hardener is used. A lower temperature will give a longer open time and a higher temperature will give a shorter open time, however, each hardener can only be used in a specified temperature range. Epoxy has a long shelf life. The technical people at West will not give a specific time, but felt there was no problem when I told them I had some hardener that was 2 years old, even though it had changed color and become thicker.
In practice, I keep 4 items from West on hand. The resin, #205 fast hardener, # 206 slow hardener, and #406 colloidal thickener. The fast hardener has a pot life of 9-12 minutes at 72 degrees and should not be used if the temperature gets below 40 degrees for 24 hours after mixing. The slow hardener has a pot life of 20-25 minutes at 72 degrees and should not be used if the temperature gets below 60 degrees for 24 hours after mixing. I find that the pot life is extended 50%-100% at a temperature of 65 degrees. Fast and slow hardener can be mixed together to give intermediate values. The parts to be bonded need to remain unmoved for about 6 hours and unstressed for about 24 hours though full cure does not occur for several days.
The resin needs to be mixed with hardener at a 5:1 ratio, either by volume or weight. West sells pumps that screw into the containers of the resin and hardener. The pumps are designed so that one pump of each gives the proper ratio, so mixing is quite easy.
In use, combine the resin and hardener in a plastic container and mix thoroughly with a wooden stick. Put a coating on both surfaces to be bonded. Then add colloidal thickener, a little bit at a time, until the mixture is the thickness of mayonnaise. Coat both surfaces with the thickened mixture and position as needed. This thickened mixture will fill any voids and make a very strong bond. Again, clamp only tight enough to hold the pieces in place. After hardening, flex the plastic container that was used to mix the epoxy and it will pop out so that the container can be reused.
I use epoxy whenever I have voids in a joint. It is very good for filling holes that will not be seen. It can be used to form a fillet next to a piece - in essence forming a ledge to help hold the piece. It is totally water proof and can be easily tinted prior to hardening. One memorable experience I had was the first time I made half-blind dovetails. I had made many through dovetails and I thought this would be no problem. I made the dovetails on a long joint with about 15 pins and tails. It took me a LONG time and then when I put the boards together, I had totally blown the joint. I was happy to be able to rescue this joint.
Acetone is the best solvent for removing unhardened epoxy and a chisel is the best tool for removing hardened epoxy from where you did not want it. The hardener is more toxic to the skin than the resin and the more exposure over time that your skin has to it, the more likely you will develop an allergic reaction. I have had a lot of skin exposure over time and I have never developed any reaction, however, I usually use gloves now when working with epoxy. If you do get it on the skin, waterless hand cleaners take it off nicely.
I must admit that when I use epoxy, I feel a sense of cheating in those situations where if I had simply done a better job the epoxy would not be necessary. However, this is simply the process of becoming an expert woodworker.
If you have any questions about epoxy, I have found the technical support people at Gougeon Brothers (The makers of West System) to be very helpful and friendly. Their phone number is 517-684-7286. There is a free technical manual and product guide that any supplier of West System should have available. They have a very helpful web site that also answers most questions.