Q&A / 

How to Glue Wood to Wood

DEAR TIM: I need to glue pieces of wood together and have never done it. It’s in my best interest to get this right the first time, so can you share some tips with me? I have both interior projects to do and some where the wood will be outdoors. I realize there are different glues to use, but am more concerned with technique. Surely you’ve glued lots of things together besides wood and had great success. Help me please. Sheri P., Arlington, VA

DEAR SHERI: You came to the right place. I’ve glued many things together over the years using all sorts of products. What I’ve discovered is that most of them deliver fantastic results if you just use them according to the directions and add some common sense along the way.

Both wood surfaces need to be clean and at room temperature for great results. Clamping is a must.  Photo Credit: Tim Carter

Both wood surfaces need to be clean and at room temperature for great results. Clamping is a must. Photo Credit: Tim Carter

You absolutely need to match the glue to the project as some glues are not formulated to withstand exposure to water once they’ve dried and cured. Other glue products are absolutely waterproof. This claim can almost always be found on 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.

If you want great results when gluing two pieces of wood, or anything for that matter, together, it helps to think on a micro scale. Imagine what’s happening where the glue is interacting with the wood, glass, plastic or metal.

Without getting into complicated chemistry or physics, realize the glue is trying to act like a burr or briar 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 on to the surface upon which it’s spread.

This means the surface should be free of all dust, dirt, oil, etc. 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.

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 - 90 F. Just read the label once again to see if there’s 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 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 made when using wood glues is the failure to clamp or apply pressure to the objects being glued. There are numerous ways to achieve this goal.

You can use hand clamps, pipe clamps, weight, screws, nails, etc. 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.

I can tell you from past experience the longer you clamp things, the better the job will turn out. I don’t mean clamp things for days, but extending the clamping time by 50 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 just around 70 F in temperature and that’s what the air temperature is where the clamping is happening.

If it’s colder than that, you absolutely need to extend the clamping and curing time to get maximum holding power.

Think about what’s being glued. 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. I’ve seen my own work fail because all the glue soaked into the end grain not leaving enough behind for a proper bond.

You can watch a wood glue video and read past columns about different wood. Just type “wood glue video” into the AsktheBuilder.com search engine.

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2 Responses to How to Glue Wood to Wood

  1. With few exceptions (epoxy, which is stronger), when properly done, most glues have a standard strength (shear) strength within 10% of each other. All these are generally stronger than the wood adherands. That means if it breaks again, it's likely to break in the wood not in the joint. So proper preparation such as clean and well-fitting joints is more important than which glue you choose. So choose the glue based on other criteria such as need to be waterproof, gap-filling properties, temperature range, open and closed clamp time, cost, etc.

    PVA glues have a "chalk temperature" of between 47F and 55F. depending upon the formulation. When the glue "chalks" it turns white as it dries and looses significant strength. So if you are doing glue ups in a cold garage, bring the wood and glue inside to warm up prior to gluing. Your spouse will not mind your using the kitchen table as a clamping table if you cover it with plastic first.

    When polyurethane glues cure, they foam up. While the foam fills a gap, it has no strength. A recent magazine test showed that the only failures of all their rests were polyurethane glue.

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