I operate a one person business building furniture, bookcases, cabinets, interior/exterior residential doors, shutters and other millwork. I was initially attracted to polyurethane adhesive due to the long open time it affords. My shop is located outside of Albuquerque, New Mexico and is situated at an altitude of 6400 feet. Humidity is almost always low in this locale and temperature swings from daytime hours to nighttime hours can be extreme. Differentials of thirty degrees are not uncommon. I heat my shop during the colder months with a wood burning stove, but even without the heating, the temperature in the shop never drops below 40 degrees Fahrenheit, due I am sure to the insulation I took care to install. During the summer months the temperature in the shop never goes above 80 degrees Fahrenheit. I furnish this information to inform you of my working conditions, so you will have a better understanding of how polyurethane adhesives have worked for me.

What follows is not to be a scientific study on the use or suitability of polyurethane adhesives….Rather, it will hopefully answer questions concerning its suitability for woodworking applications. I have used polyurethane adhesives in various joinery applications for at least two years. During this period of time, I have found it to be very useful when multiple components must be assembled and polyvinyl acetates and similar faster setting adhesives would not serve well due to their relatively short open times. For the sake of brevity, I will refer to polyurethane adhesives as “poly or polys” for the remainder of this article. While I am a fan and advocate of polys, I urge anyone contemplating their use to satisfy themselves as to the value and practicality of this material. What has worked for me and others may not work for each individual user due to climatic and material conditions at your shop/work site. So, before opting for polyurethane adhesive for that expensive piece or pieces….Test !.

Some Observations On Polyurethane Adhesives

As pointed out above, polys in general have a relatively long open time when compared to polyvinyl acetates and similar faster setting adhesives. Because polys use moisture present in the atmosphere and in the material being assembled, clamping and cure times can vary. Higher relative humidity and higher moisture content in the wood itself hastens cure time. Ambient temperature and the temperature of the materials also have an effect on the cure rate, but probably not to the extent that moisture does. Some users of polys, including myself, wet/moisten the components to induce more rapid cure times and to insure there is enough moisture to allow the adhesive to cure effectively. How much moisture to add is best left to experience, however, I have found if the wood changes to a darker shade from the application of moisture, it is adequate. You should not be overly concerned about adding too much moisture as most suppliers of polys state the adhesive is effective on wood which has a moisture content of eight to twenty percent.

Polys seem not to mind if the edges or surfaces being joined are milled to a smooth finish. I have joined components straight from ripping on the table saw and found the wood failed and the glued joint held fast when the joint was stressed. One supplier, (Excel) even states in some of their printed material surfaces are best left in a “roughened condition”.

Before the poly cures, any excess can be removed with acetone, denatured alcohol, or mineral spirits. Fully cured poly is easily removed with a wood chisel or scraper and it can be sanded without filling the sanding material with residue. Before you get the glue bottle out….Unless you are excessively neat and careful, you should plan to wear some sort of cover or barrier on your hands. I use latex gloves and have found if I am careful, I can use the gloves on several operations (no pun intended). For those who are allergic to latex, try rubber or perhaps one of the barrier creams will work.
If you choose not to cover your hands and you get the poly on your skin you will have to use one of the solvents listed above to remove it or you will have to wear it off over a period of three to four days. Once the poly dries, soap and water offers no remedy for the brown stains which will appear on your skin. I believe all of the mentioned solvents can be absorbed through the skin, therefore I suspect the less applied to the skin for cleansing, the better.

While I have no desire to be controversial or stray from the accepted path of acceptable woodworking and adhesive practices, I have used polys for plate joinery and continue to do so to this date. Experiments I have participated in as a principal at Sandia National Laboratories and simple shop tests I have conducted on my own convince me the method is appropriate and long lasting. For some information concerning the results of the Sandia tests you might want to refer to articles in American Woodworker (issue #50) and Fine Woodworking (issue #118). I have been and am still attempting to get a comprehensive posting of the tests results posted on Sandia’s website.

1.3-Polyurethane-Adhesives

For the present, I can relate to you my personal experience in using Lamello S-6 bisquits and polyurethane adhesives in the construction of interior and exterior residential doors. Eight doors which I made from Ponderosa pine (one exterior, seven interior) have cycled through two winters, 1 1/2 summers, two adults and four rug rats. I have the opportunity to examine the doors on a weekly basis and have found no separation at joinery locations or any warping, bowing or other problems. The exterior door has been subjected to driving snow, rain, and dust. Although I have made thirty-four other doors using the same methods, they are not available to me for constant observation, but no client has indicated any problem with any of the thirty-four.

I have conducted what seems to me to be endless shop tests of polyurethane adhesives and woodworking materials. One of my friends, tiring of my never ending daily reports to him started calling me “The Guru Of Glue”, I sort of liked the title, but turned the phrase a little and changed it to “The Gluru Of Glue”. Please don’t infer from this account that I fancy myself an expert with polys or any other adhesives, just a little aside and an attempt to impart some humor in this article.

Many of my tests in the shop involve trying to determine what happens to a bisquited joint when glued with poly. By way of explanation, after cutting the slots and applying the adhesive in the slot and on the surface grain of the components, I dunk the plates in a container of water and remove them right away. I then place the plates in their slots, assemble the components and clamp for the appropriate period of time. I try to plan my assembly and glue-ups so I can leave everything clamped overnight. This generally works for me, but may be a little overkill for those in higher humidity locations. After the adhesive has cured, the clamps are removed and I subject the joint to varying degrees of stress. Sometimes the assembly is placed in a vice and struck with a hammer. I have found the joints to be on a subjective par with joints assembled with PVA adhesives. More importantly, some of the samples are sawed across the joint location , so that the space between the bisquit side and the wall of the slots can be observed. My fifty-six year old bifocal aided eyes have thus far, be unable to discern a glue line, much less unfilled space. A 6X power, cheap magnifying glass, also fails to reveal any space at the location. The excess space at the ends of the bisquits are usually filled with cured adhesive. When I try to remove the bisquit portions by levering with a scratch awl, I find the plate material comes out in very small portions, sometimes bringing some of the wood from the wall of the slot with it. This compares to the same general results with joints assembled with PVA adhesives.

In addition to the above tests/experiments, I have made sample joints using various poly products to edge join and butt join various materials. One of the most striking experiments involved gluing two pieces of Yellow Poplar end grain to edge grain. Both pieces were five inches wide and eighteen inches long. After an overnight cure, I placed a corner of one of the legs of the angle formed by the components on the floor and placed all of my weight on it. The joint did not come apart from my weight and a repeat of the process by two of my bigger friends failed to fracture the joint. Eventually, I became tired of walking over and around the piece and whacked it with a 16 ounce framing hammer. The joint finally fractured but not at the glue line. Approximately one half to one inch of wood came away on one of the components. No mechanical means of strengthening the joint were utilized. Just polyurethane glue and wood.

I have become email buddies with Mr. Don Watland of the Phoenix, Arizona area. Mr. Watland runs a one man operation (Watland Design) and makes traditional and contemporary furniture. My first email contact with Mr. Watland came after I saw a post of his on the user group “rec.woodworking”. In his post Mr. Watland related he had been a user of polyurethane glue and bisquit combinations for over a year. His experience with the glue and bisquits closely tracked mine, so I asked him to share some of his methods and results with me and the readers of this article.

Mr. Watland has advised me he also wets the bisquits before inserting them in the slots. He relates that after wetting, the bisquits quickly swell to fill the slots to the point they can not be easily repositioned. Mr. Watland’s experience had up until recently been with Gorilla only. He has recently obtained a sample of Excel regular, but has not put it through it’s paces. In addition to edge joining plywood, Mr. Watland has bonded acrylic sheets as thin as 1/8″ to wood members with slots or rabbets to receive the acrylic for 90 degree and “flat” joints, he has bonded Kortron to itself and to solid wood, and he glued up an “L” shaped bar top from sections of 3/4″ ash veneered plywood which was edge joined in several places. Mr. Watland states his procedure for gluing the Kortron was to sand the surface of the Kortron with 80 grit sandpaper to the point that the shiny/slick surface of the Kortron was “roughened”. His bar top was able to hold his weight when he cantilevered it over his workbench top. Mr. Watland still uses and likes white and yellow glues, but does reach for the polys when he needs extra assembly time or there is any question about that aspect of his operation. He has also found it performs extremely well with woods such as ebony and rosewood.

Within the last few days, I used Excel Regular and Excel Express to glue up two samples of white Melamine (melamine to melamine), I applied moisture to one of the surfaces of each sample and allowed it to cure overnight, (approximately 12 hours). When I placed the samples in my vise and struck them with a metal hammer the joint fractured into the particleboard substrate all along the glued portion. Both samples displayed the same results.

One final glue story and I’ll provide some new information on new products and the opinion of at least one of the present suppliers of polys…..I cut sized pine plywood pads for all of my bar, pipe, and similar clamps, applied poly to the wood, and placed a pad on each of the metal heads of each clamp and then slid the pads together and applied light clamping pressure until the glue was cured. You don’t need a lot, just enough to cover the surface, (be careful not to get any glue on the face of the pads, you don’t want to lock them together , do you?). This was done over a year ago and I have not lost a pad to this date. Be sure to raise the plywood high enough so it doesn’t touch the bar or pipe or it will not move smoothly in operation. If your pad becomes worn/damaged you can remove it with an old chisel or other sharp object.

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I have mentioned a product currently called Excel Express. This product is a thixotropic gel. My understanding is that thixotropic means, it liquefies when subjected to motion. If such is the case this product lives up to the description as when it is brushed or otherwise moved it does liquefy. Unlike “regular” polys this product cures for clamp removal in my shop in less than thirty minutes and starts to foam almost immediately. My impression is, it is stronger(my subjective opinion) and more gap filling than regular polys. This product comes in a tube similar to a regular caulk tube and is used with a caulking gun. At first I found this methodology to be cumbersome, however after conducting several shop tests, I found it did not slow me down when I applied it to bisquit slots or regular edge or other joints. I also worried that the glue in the tube would dry out like caulk often does even when a cap or wire nut is placed over the spout. I actually found if the excess glue which might come out of the spout is allowed to cure, when you get ready to use the adhesive again, you just remove the cured portion and fresh, uncurl glue presents itself. I was lucky enough to acquire sufficient of this glue to use it on some of my projects. In all cases it exceeded my expectations. This product is poised for marketing in late summer and will probably be more expensive than the regular polys. I am not positive what stores will be carrying it, but I would suspect Woodworker’s Supply, Trend-lines, and other stores which presently carry Excel product will opt to carry the Express.

I recently purchased a product from Custom-Pak Adhesives labeled as RPA. I believe the letters stand for Reactive Polyurethane Adhesive. Open time for the product is stated as 15 to 20 minutes and clamp time about 90 minutes. Shelf life is given as” up to four months in a tightly closed container in a cool dry environment”. I performed my “standard” double bisquit butt joint test with two sample joints. Left to cure for about 12 hours this product performed very well. When sawed apart and examined with my “magic” cheap magnifying glass I could discern no glue line and all of the excess slot space was filled with cured adhesive. I would have liked to have performed additional tests with this product, however the deadline for this article caught up with me and I was not able to do so. I was not asked to, but I did provide Custom-Pak with my short evaluation of the product. I also indicated to them I thought the product was a little pricey for adhesive with a shelf life of four months. The 18 ounce container was purchased for eighteen dollars. Upon further thought and in retrospect, I believe I might have been hasty in complaining about the price and the shelf life. For one thing…All polys go a long way and you do not have to apply the products to both surfaces…For another I would use the 18 ounces before the shelf life expired. As a final observation concerning RPA, I noted it cures to a white color very similar to Excel Express.

I believe that any of the liquid or gel poly products I have worked with or tested are very good products. I’m not here to endorse one over another. I know my shop tests are not scientific and I have used the product in some applications which are not recommended.
Examine the results I have tried to present to you, perform your own evaluations/tests and decide for yourself if polys are a real option for you. In some applications you’ll decide polys are overkill, but you may find a new and efficient partner to help you in some operations. Finally, I have not addressed the issue of whether or not the poly products are “waterproof”. Some labeling and product literature indicate it is. I don’t believe all of the product, if any, currently on the market in the United States has been subjected to strict independent laboratory testing to determine to what extent it really resists water when cured. Perhaps in the not to distant future, tests will be performed which will answer the question. To paraphrase, a comment by Mr. Sammy Mayeaux of Ambel Corporation, (Excel Distributors)…. What we see now in polyurethane adhesives is just the tip of the iceberg!

In closing I would like to acknowledge the help given to me by Mr. Don Watland of Watland Design, Mr. Sammy Mayeaux of Ambel Corporation, Mr. Jeff Pitcher of Custom-Pak Adhesives, Inc., the folks at Gorilla Group, and Mr. Eddie Sirotich of this website. Certainly any lack of content and writing ability is my responsibility and fault. I hope if you have questions or comments (be gentle) you will present them.