So I’ve always wanted someone else’s opinion regarding gluing and clamping boards together so here goes.
1) When gluing edges or surfaces of boards together is it necessary to glue both sides of the board. Or can you glue one side with an adequate amount of glue and only do one side.
2) When clamping how much pressure should you apply. Applying too much seems to always bow the piece regardless of how careful about placing/rotating clamps. Some say too much pressure squeezes out too much of the glue. However you want enough pressure to make sure the boards adhere together.
Replies
I always spread glue on both parts. It gives a much stronger bond than when glueing one part only. Much ofvthe time tgat us strong enough. Some times, it won't be. And it takes very little time and effort to put glue on both halves.
I'm not sure how you are bowing pieces with clamps. But I clamp hard enough to bring the pieces together, solidly. Figuring out how much glue to apply is the hardest part, requiring experience and repetition. Clamped well, a very fine bead of glue should squeeze out, but not enough to run or drip.
The joints in table tops should not get much stress. They are flat and normally supported underneath. That being said, I've always only glued one side. In my opinion, if you have squeeze out then there is ample glue in the joint. As to bowing (such as with a table top), I normally clamp a caul over and under the wood, perpendicular to the planks, to hold it flat. Then I clamp the joints as tight as I can. I've never done side by side comparisons, I've never had problems with joint failure using this method.
Thanks for your response. I may have to try the cull method on my next project.
I recently built a barn door, using 2x8 T&G spruce, and had the usual cupping problem. I used a straight 2x4, laid on edge, at each end, clamped perpendicular to the T&G as a "strong back" to keep the door flat.
You should always wet both surfaces with glue. A quick way to do this is to stand one piece on edge and with a bottle, squeeze out a very heavy bead of glue on it. Flip it down and in contact with it's mate, then slide the two pieces back and forth against one another to distribute the glue. This will wet both pieces prior to clamping and is much faster than brushing an even coat on the two.
Regarding clamping: I never use cauls, a total waste of time. Proper preparation and clamps will insure a flat panel. Joint a face and both edges prior to glue up. If I can get the panel through my planer after glue up I will leave the thickness a little heavy, if not, plane the boards “net” and use dominos or biscuits to index the faces level, absent these you can use a dead blow hammer to level the joints after getting clamps on and under a bit of pressure.
Proper clamps are a must. Pipes are for plumbing, they will bow the work. “I” beam or Record style sash clamps are best. Bessy clamps are ok too but Jorgenson are the best IMHO.
Place the jointed or flat side of the boards down on the saw horses, lightly clamp them together with a clamp at each end , then take a wet rag( I keep a 5 gal pail of water with a rag in it in the shop for this purpose) and wipe off the glue that squeezes out, flip them over in the clamps and do the other side, then flip them back to the original position.
I put a small piece of wax paper over each joint where the clamps sit to avoid staining the wood black. Alternate the clamps top and bottom every other one and keep the bar in hard contact with the face of the panel. If every thing is properly milled it is hard to go wrong. Pull bar clamps up snug and then give it a bit more. With Besseys get them as tight as you can by hand.
Good Luck
Rob
PVA glues are mechanical glues. The polymer chains wick into the wood a little and then set hard as the glue dries. Once dry, they resist wetting and further penetration of glue, becoming entangled with each other and the wood fibres permanently, thus making a very strong joint.
In order for a good bond to form, the joint must have enough glue in it to allow some to penetrate both surfaces and also leave enough to join the two together. No knots, no stick...
Under most circumstances, applying glue to only one surface is sufficient for this because it is applied reasonably thickly and the space between pieces is tiny - there is ample glue to do the job.
That having been said, applying glue to both surfaces ensures that the surface is properly coated and there is no risk of a pocket getting missed. PVA can bridge very tiny gaps but will not do so if there is insufficient pressure. A small ding in an edge can leave a gap too large for the polymer chains to bridge effectively.
In general whether you have a perfect joint or not makes no difference as the glue joint is so much stronger than the wood it will not part even if it is not at maximum strength so it is not really necessary to glue both sides for most applications.
It may be more important to glue both surfaces where the joint is small, the surface is very porous (such as end grain, or woods like cedar), maximum strength is required or where the joint may become exposed later and any pockets will be visible, such as in segmented woodturning.
Porosity is important. PVA will only work with porous surfaces, so you can't seal the joint first. Once PVA is well-set there is no point in adding more. Epoxy is the way to go there. If the joint is very porous though (end grain) a lot of the glue may wick into the wood so some experts advise a first application to both surfaces, then a second a few moments later to ensure there is enough in the joint for a bond. Personally I believe that if you need to do this then you probably need to redesign the joint...
So in summary, whilst the science supports applying glue to both surfaces, to all practical intents and purposes, it is only a significant issue where you are pushing the limits of what wood and glue can do.
Just remember that glue is very cheap. Don't be a miser...
I worked full time for 13 years building custom furniture, including several tables, cabinets, desks, etc. With very few exceptions, I applied PVA glue to one side only of the joint then clamped them firmly, using cauls where necessary. I never had a glue joint failure. 'Nuff said.
Just apply the glue to one edge, but a fairly heavy spread. If after applying the clamps glue is running all across the face of the glue up, you've applied far too much. You're looking for tiny beads of squeeze out right at the joint line, all across the joint.
Glue is really strong, so if the panel has been properly jointed, the ends aren't open, even if you're a little stingy with the glue it'll hold. Better this than a huge mess and potential problems with glue stains telegraphing through the finish. This is especially important if you plane the individual boards to finished thickness before gluing the panel -- you have no material left to plane off to remove glue stains. It's better to process the individual boards as minimally as possible before gluing the panel. Then, the entire panel is worked as a unit as if it were a single wide board that came from one of the huge trees of yore. In this scenario, you'll remove enough material that glue stains are essentially an impossibility.
I went to a seminar given by the director of tech support for Titebond.
1. He said it did not make any difference whether you apply glue to one or both surfaces. Either was acceptable.
2. He debunked the idea of too much pressure "starving the joint." In their tests with red oak, they could not over-tighten joints with the usual clamps. They found that the joint were slightly stronger with more pressure.
To avoid cupping though, I always alternate clamps, one up, one down, across the joint.
As they say, though, "Opinions are like (um, rectums) -- everyone has one." I trust testing and data.
All seem to be offering you good advice. I have heard this debated on podcast. As Rob SS says, the consensus (or maybe most convincing opinion) seemed to be modern glues, even plain yellow, allow for one side. For panel glue ups or butting along grain, for rabbets or gluing stacks side to side, have adopted fully covering one side via spreading ample glue then putting quick thin bead on other side and giving it quick smear. This assures I don’t skimp. I always clamp enough to draw pieces together plus tad bit more. For shelf or cabinet dados, miter joints, or any other end grain, I always apply to both - end grain sucks up glue and this assures good coverage.
For short- or end-grain in joints, try Titebond's No Run - No Drip or Quick & Thick. At that same seminar above, the presenter glued poplar end grain of 1x2 with that glue and fifteen minutes later stood on it without breaking. He could eventually break it, but it took some effort.
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