Gluing dovetails and finger joints
PVAs tack up very quickly, especially on warm summer days. This can be an advantage in many cases but on complex assemblies it can be a disaster, as the glue will grab before the joints can be properly pulled up. Try plastic resin glue. It will give you more assembly time. Itís cheap, strong and almost waterproof (type 2) and it wonít clog sandpaper.
Gluing dovetails and finger joints are really two different things based on the geometry of the joints. Both are mechanical joints involving the end grain of wood, but that is as far as it goes.
Finger joints are an end to end press fit that creates a continuous relationship of the grain between two pieces. A dovetail is an interlocking fit with a perpendicular (cross grain) relationship.
In years of both hand cutting and machine cutting dovetails, I have never had a problem with PVA glue unless I was using an odd species of oily wood. Early on when I cut dovetails too tightly, most of the glue would be squeezed out when assembling the joint. This is why most people tell you to have a paper's thickness of space in the joint.
Finger joints are another matter. There are many adhesives that work well in finger joints. The main issues are the viscosity of the adhesive, the fit of the joint, the way the material is pressed together and the species of the wood.
Unfortunately, unless you are in production and can spend a lot on finger joint tooling and/or a finger jointer, it is nearly impossible to get tooling that allows you to easily adjust the fit. Because of this we need to look at the other issues.
In this discussion, viscosity may be the main culprit. An adhesive that is too viscous (thick) will create a hydraulic action when the wood is pressed. This will force the joint back open. Most manufacturers can provide you the right adhesive if you tell your rep what you are doing. We have many customers using Franklin's Multibond 2000. It has the appropriate viscosity for many finger joint applications. There are of course equivalents produced by other manufacturers, but Franklin tends to be a very accessible company for support.
Species issues make this even more interesting. Softwood tends to be more forgiving because the wood will crush slightly in the pressing application. Thus, less chance to hydraulic.
A finger joint is essentially a mechanical joint. If cut with no tolerance for adhesive and pressed, it is actually very strong if used within certain orientations. However, this is incredibly impractical. Pressing gets the joint as tight as it can be. It creates the frictional heat that starts the adhesive curing. Without a good press it is hard to get a successful finger joint.
Another note about adhesive. Be careful. It is important to know what the use of the material will be in the final application. Water-resistant does not mean waterproof and different adhesives are needed for different uses.
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