Homework: How to glue wood on wood
CHOOSE THE RIGHT GLUE
It's important to match the glue to the project. Some glues are not formulated to withstand exposure to water once they've dried and cured. Other glue products are waterproof. You can usually find out by reading the label of the product. If you don't see any wording on a glue label stating that it's waterproof, assume that it's not.
CLEAN THE SURFACES
If you want great results when gluing two pieces of wood -- or anything, for that matter -- it helps to think on a micro scale. Imagine what's happening where the glue is interacting with the wood. Realize the glue is trying to act like a burr or brier that might attach to your clothes as you walk through the woods. Dried glue has a structure that has miniature hooks, barbs, etc. that try to latch onto the surface upon which it's spread. This means the surface should be free of all dust, dirt and oil, and it helps if the surface is somewhat rough. The roughness creates more surface area for the glue to grab onto, and it creates more places for the tiny hooks and barbs to attach themselves.
CHECK THE TEMPERATURE
Temperature is also a consideration. Most glues you'll find at stores have a water base and should be used at temperatures that range from 50 to 90 degrees F. Again, reading the label will tell you if there is a preferred temperature range.
The common yellow glues that are water-based work as the water evaporates or soaks into the wood. As the water leaves, what's left behind is the actual glue that does the work. Think of the water as a delivery vehicle. Some other glues have a totally different chemistry and require the surface to be wet or that there be significant humidity in the air for the glue to react and bond. Once again, read the label and do what the manufacturer says with respect to wetting the surfaces slightly.
Perhaps the most common mistake people make when using wood glues is failing to clamp or apply pressure to the objects being glued. There are numerous ways to achieve this goal, including hand clamps, pipe clamps, weight, screws and nails. The object is to do whatever is necessary to squeeze the pieces of wood together for as long as the instructions say to apply the pressure. Clamping time can be as short as 30 minutes or an hour. The longer you clamp things, the better the job will turn out. We don't mean clamping things for days, but extending the clamping time by 50 percent to 100 percent of the time mentioned on the label is not a bad idea. Remember: The clamping time was probably set assuming the objects being glued are a temperature of about 70 degrees. If it's colder than that where you are doing the work, you need to extend the clamping and curing time to get maximum holding power.
CHECK END GRAINS
It's important to think about the specific qualities of what's being glued. For example, the end grain of wood pieces will readily soak up glue. If you're gluing end grain, spread some glue onto the end of the piece and move it around with your finger or a stick. Wait about two minutes to see if the glue soaks in. If it does, add some additional glue before you clamp the pieces together to ensure there will be enough left at the joint to do the job.