Different Glues - Different Techniques

In a recent article, “Knowing Your Glues” (Canadian Woodworking, Feb/Mar ’06, Issue #40), I reviewed some of the more common types of glue used in woodworking. I mentioned that the most commonly used glues are white and yellow PVA. Personally, I use yellow glue for the vast majority of my needs. I use Lee Valley’s 2002 GF high solids PVA for more difficult glue-ups where I need a little more open time. For glue-ups where perfect wood-to-wood contact is unlikely, and for bent laminations, where I need high strength and creep resistance, I use 24 hour epoxy.

PVA glue bonds chemically within the cell structure of the wood. You’ll hear about ‘glue-starved’ joints where joints fail because of too little glue, but this is misleading. Glue-starved joints happen when there isn’t enough glue to sufficiently wet both sides of the joint. Glue must penetrate the wood cells on both sides before it can chemically join the two parts. However, there is no requirement for excess glue to remain in between the parts. In fact, PVA glue bonds better with no glue thickness between the layers! You need good wood-to-wood contact and, preferably, a zero glue-line thickness. When using PVA glues there is no such thing as too many clamps and/or too much pressure.

Epoxy glue, on the other hand, bonds on top of the wood, not within the cell structure. It requires some glue-line thickness for proper strength. Instructions from some epoxy manufacturers suggest an ideal glue-line thickness of .003″ and warning that overclamping will squeeze out too much glue and fail to achieve an optimal bond. So over clamping is a real possibility with epoxy.

Preparing Your Stock

For the above reasons, precisely made joints that fit well together, are best glued with PVA glue. Edge joints, such as when gluing together boards to make a table top or any solid wood panel, also need to fit extremely well. My personal rule is that the joint must close tightly with only hand pressure. You might counter that you have all kinds of powerful clamps in your workshop and surely they have enough power to close your ‘sprung’ joint. But what happens when you remove the clamps? Can you really expect the glue to work that hard 24 hours a day over the long term?

The best gluing surface for PVA glue is a freshly planed one. While a hand planed surface might be best, it isn’t realistic in most cases. A freshly jointed or planed surface is. Whenever possible, try to mill your boards just before glue-up. Fresh cutting opens up the pores of the wood, allowing it to absorb the glue. Left for several hours, the pores close up slightly, almost like a fresh cut scabbing over.

When joinery is less than perfect, 24-hour epoxy is a better choice. A slightly sloppy mortise-and-tenon joint can be glued with epoxy. However, I would rather glue a piece of veneer to one of the faces of the tenon and then assemble the mortise and tenon with PVA glue than glue up a sloppy joint using epoxy.

Applying Glue

The application method isn’t as important as using the right amount of glue. You can apply glue by brush, roller, scraper or whatever other method works for you. Some woodworkers apply PVA glue to narrow edges with their fingers. My favourite tool for applying glue on narrow edges is a European round brush with stiff tapered bristles. For wider edges or the faces of larger boards, I use a shoe brush. When gluing large panels together face-to-face, I’ll resort to a rubber roller, although it tends to push the glue around rather than rolling it, as PVA glue is very slippery. You could also use a small paint roller.

I apply PVA glue to one side only in the interests of time. But remember that this is only sufficient if you bring the two parts together very quickly, allowing the glue from one side to transfer to and penetrate into the other side. If you let the part with the glue sit for a few minutes, it will skin over and won’t allow proper penetration of the second side.

You want to see sufficient squeeze-out with both edge-gluing and face-gluing. Too much glue is better than not enough, provided you use enough clamping pressure to push the parts down to nearly zero glue-line thickness (assuming PVA glue). I like to use a putty knife to pick up the excess glue and wipe it on a rag as I go. One of the biggest mistakes people make is to wipe the excess glue with a wet rag. The water thins the glue, driving it deeper into the pores of the wood. This leads to nasty surprises when it comes to staining and finishing. Where you don’t have a lot of squeeze out, you can wait 30 minutes for the glue to slightly harden, then scrape it off with a sharp chisel or card scraper. When gluing closed joinery (such as dowel, biscuit or mortise-and-tenon joinery) there should be no glue squeezeout. Apply the glue in the hole, biscuit slot or mortise. With experience you will learn to apply the right amount. There is no excuse for excessive squeeze-out with these types of joinery. While chisels, scrapers and sandpaper will help with squeeze-out, they are never foolproof or guaranteed, and are very time consuming.

Clamps

I own about a dozen high-end parallel jaw clamps. I call them ‘finesse’ clamps.

They do a nice job on boxes and drawers, or any high-maintenance glue-up requiring a lot of precision. But these clamps also have a number of draw-backs, including high price, too little strength in many applications, and less accurate parallelism than manufacturers claim. On the latter point, I can’t tell you how many drawers or boxes I’ve glued where these clamps simply don’t close the joint tightly across the entire face of the clamp, requiring me to insert a shim (like a business card) on the far side. As for strength, I’ve had many joints that wouldn’t close fully with a parallel jaw clamp, but closed with ease when I substituted a pipe clamp, like a Pony #50.

For edge-gluing, nothing beats the simplicity and power of a pipe clamp on 3⁄4″ gas pipe. Stay away from the galvanized pipe because the tail piece often slips. Ordinary, less-expensive black gas pipe is all you need. If the boards are 21⁄2″ to 3″ wide or wider, one clamp every 8″ or less is a good idea. They should be spaced even closer if the boards are narrower, or when face gluing (gluing boards face-to-face instead of edge-to-edge). In reality, a huge amount of pressure is required for proper edge and face gluing, so don’t skimp on clamps. High pressure isn’t required due to poor joinery, but rather to squeeze out all excess glue and give you that zero glue-line thickness you’re after with PVA glue.

Large C-clamps and strong F-clamps (particularly those with swiveling handles to give you extra torque) are also workshop mainstays for all kinds of gluing.

When gluing joinery, like dowel, mortise-and-tenon, etc, note that once the joint is closed, extra pressure does nothing but bend the parts. The tightness of tenon to mortise is completely dependent on the accuracy of the fit. Cranking down on your clamps even after the tenon shoulders are touching does not help.

Dry Time

Most manufacturers of PVA glue suggest that 30 minutes of clamping time is sufficient. However, cooler temperatures and higher humidity levels extend dry time. Even in my fully heated and humidity controlled workshop, I don’t feel comfortable unclamping in under two hours, except perhaps for minor gluing of mouldings or other detail-work. For really important glue-ups, like the final assembly of a cabinet carcass, I feel better leaving the project in clamps overnight.

Remember that even though an hour or two might hold the wood together, full cure isn’t achieved for 24 hours. So you shouldn’t stress the glue-up for that period of time. Don’t unclamp a panel glue-up 30 minutes after gluing and start running it through the jointer and planer right away. Go on to something else and come back to re-mill the next day.

For 24 hour epoxy, full cure isn’t achieved for 7 days. I keep the glue-up in clamps for 48 hours and don’t start working the joint for two or three more days.

Be Patient

Like most things in life, patience is required. Take the time to mill smooth surfaces. Blasting across your jointer might make you look like a pro, but tearout and widely-spaced mill marks don’t help your gluing surfaces. And make your joinery tight. When you know that a joint is too loose, don’t just close your eyes and cross your fingers. Either repair the loose joint or use a different type of glue that can handle the situation.

In time, your skills will improve, from your joinery to your gluing and then on to finishing. Stay tuned for some more specific gluing tips in my next article.