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From the pages of Fine Woodworking Magazine Gluing and Clamping Strategies Lots of tape, lots of cauls and lots of clamps make glue-ups easy
In this article I'll outline a few common gluing situations a woodworker has to tackle and provide a few tips that, hopefully, even the seasoned woodworker can use. It's easy to test your gluing system If a glue joint fails, the first suspect is the glue itself. But the glue is seldom the real problem. Modern glues are so effective as to be nearly foolproof. More often than not, operator error is the problem, not the glue. You can easily test both the glue and your gluing system to increase your confidence. Simply glue up a sample, let it cure, then break it apart. If the sample breaks at the joint, you have a problem with the glue or the joint. If the wood does not break along the glueline, the test demonstrates that the joint is stronger than the wood, which is what you want. If necessary, keep modifying your system until you achieve satisfactory results. Glue has a limited shelf life On those rare occasions when the glue itself fails, either the type of glue being used is unsuitable for the job at hand or the glue has exceeded its shelf life. Remember, some glues have a very short shelf life -- as little as three months in some cases. One year is more common, although it varies a great deal. One thing is certain, though: If you are still using that bottle of glue you got from Uncle Harry before he died a few years back, it's at least suspect. Do yourself a favor and buy a new batch. I recommend buying small quantities of glue, as much as you'll likely use in a few months, from a place where the turnover of merchandise is high. That way, you'll run out of glue before it gets a chance to spoil. When you bring home a new bottle, write the date on the bottle with a felt-tipped marker. Relatively speaking, glue is cheap. When it gets old, throw it away. Many glues are susceptible to spoilage from freezing or excessive heat. It's important to keep track of temperature for both storage and actual gluing. Different operations require different types of glue Before you even buy a bottle of glue, read the label. Glue manufacturers want your glue-ups to be successful. They make a point to tell you exactly what you need to know right there in the fine print. If the label says the glue is unsuitable for use around moisture or under stress, no doubt they mean just that. Lamination bending, for example, puts the glue joint under constant stress. Yellow glue, tough but flexible, is unsuitable. Epoxy or plastic resin is a better choice. Some glues, the polyurethanes for example, like dampness, but most glues do not. Another reason to read the label and do some testing. Well-fitted joints are a must Clean, dry, well-fitted and porous edges glue well. If the mating pieces do not fit well enough without clamping pressure, perhaps the work is not yet ready for gluing. Joints ought to fit naturally without being forced by oversized clamps. If you have to force the joint together, you're asking a lot of your glue to keep it that way once the clamps are released. Porous edges allow the glue to ooze into the pores and strengthen the joint. If you make a very slow pass over a dull jointer, chances are you'll pound the fibers of wood together instead of cleanly slicing them. This forms a glazed, nonporous surface that won't be suitable for gluing. Cauls and dry-fitting are the keys to success Moving too slowly may well be the most common cause of glue failure. Very quickly -- especially in warm, dry weather -- the surface of fresh glue will form a skin, a sort of blister. This blister prevents liquid glue from adhering to the other surface. If the glue skins over before the two mating pieces come together, you might as well call it a day. Sure, you can scrape off all of that excess glue, but the residual glue will seal the surface. The edges will need to be remilled, not just cleaned. One sure way to reduce the time it takes to get something clamped up is to practice. Especially if the gluing operation is complex or large, a rehearsal helps ensure that you have all of the clamps you need and that all of the little blocks and pieces of masking tape are at hand. It's a good idea to dry-fit and clamp up the project entirely, as if you had spread the glue already, to make sure one last time that the pieces actually go together. Keep in mind that the addition of glue in the joints will change the operation a great deal. It lubricates the pieces just enough for them to move around when you don't want them to. Then the glue begins to set up and grab the pieces, holding them in place just when you want them to move. But these problems are easily solved. The key for almost every gluing situation is to use clamping cauls (see Edge gluing). Simply put, clamping cauls hold boards in alignment while other clamps close up the joints. Clamping time -- The amount of time a project has to remain clamped up depends on temperature, humidity and the complexity of the project. It's important to follow the directions on the label of the glue you're using. If the label says clamping time is an hour, give it an hour. If the label says overnight, wait until the next morning to remove the clamps.
Cleanup -- Gluing over butcher paper or newspaper certainly saves you from having to do a great deal of cleaning up. Wearing vinyl gloves not only keeps hands clean but also helps you avoid contact with toxic chemicals.
Along a glueline, I want to see at least a thin bead of glue. When the glue has set up in the joints and the clamps are removed, the excess glue should still be soft and pliable. This is the perfect time to remove the excess with a scraper. I never wipe up glue with a wet rag, because the added water will raise the grain and the finished surface will be uneven.
I do my best to use a simple and quick system for gluing. Once the glue has been spread, I use every trick I know to speed it along. I always follow the manufacturer's instructions to the letter. When in doubt, I call the manufacturer on the phone. Manufacturers have always seemed more than happy to discuss individual situations. I have a habit of testing my glue joints constantly. If I trim a glue-up to length, I take the scrap piece and snap it over the corner of the bench to make sure the joint is reliable. I am always happiest when it breaks 1/2 in. away from the glueline. Your confidence builds with every test. If you adopt this habit, you'll soon be confident in the boards you glue together. You'll sleep better, too.
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