Any validity to this glue-up advice?

[Brian W Evans]

My son wants to build a bow and we found this video series, which seems pretty good, except (possibly) for this gluing advice I've never heard before. He says that you need to put on a thin layer of glue, let it dry most of the way, then put on a thick layer before gluing pieces together (long grain to long grain). He says this is necessary because the wood will soak up the glue and starve the joint. Finally, he says that too much clamping pressure will also starve the joint. He's gluing purpleheart to maple using Titebond II.

Here's the video: https://www.youtube.com/watch?v=5IahNAJt79I (glue-up advice starts at 1:25)

I've never heard or read that you need to, essentially, size a piece of wood before gluing up, nor have I ever done it. I've also read countless times that, within the general limits of woodworking clamps, more clamping pressure means a stronger joint.

Anyone have any thoughts on this? I don't plan on changing my glue-up practices, but I'm curious about the origins of his ideas.

[Steve Cowart]

I have heard people mention "Starving" the joint of glue with too much pressure before. I have always used K-Clamps with 1500 pounds of pressure and as many as I can fit. I have never had problems with a joint before. Even if I slice off a paper thin crosscut the wood will break before the joint. I am hardly an expert, so I am curious to hear what others think as well.

Good luck on the bow, sounds like a fun project.

[Andrew Hughes]

I've never been able to squeeze out all the glue of a joint that I properly covered with glue.
I don't use that technique of coating and recoating Ive heard of it .
Do some testing on scraps see what works for you.
If you have time report back your findings.

[Mel Fulks]

I'm guessing that could not possibly be good advice unless.....it is a purist indigenous people technique with wig-wam made glue.

[Sam Murdoch]

This can be a useful technique on fast absorbing END GRAIN but for long grain to long grain the nearly dried glue will just add an unnecessary thickness to the glue line. Some people will tell you that purple heart needs an alcohol or acetone wipe before gluing but I have never had any issue with it just being glued directly to itself or non oily woods such as maple.

Clamping pressure can certainly be overdone and result in squeezing out all the useful glue. Most woodworking clamps have long dowel type handles - rather than wheels or fat knobs for two hand grips. Kind of limits the amount of pressure one can apply. I call it the white knuckle test. If your knuckles are turning white and you are gritting your teeth and squinting your eyes as you tighten your clamps - you are OVERDOING IT Steady pressure, properly sized pads and enough clamps to keep the joint tight in its length (a variable) is more than good enough. Drying time and non stress time are just as important in the equation if you have clamped properly.

[Nick Decker]

I've heard the two-layer glue method called "priming" the joint, and I use it for the end grain on miter joints. I think using it for long grain would just result in more ooze to clean up.

[Steve Schoene]

The advise not to "over clamp" has been around a long while, but data from maker of PVA glue--Titebond--shows otherwise given the recommended clamping forces they publish. I've read reports of experimental data that showed PVA joint strength increased as clamp pressure increased up until the point there the wood begins to be crushed. Epoxy appears to be an exception and does prefer more substantial glue lines that maintain strength.

[Bill Dufour]

When other kids glued bows in high school shop class they used a cut out jig and a length of fire hose with air pressure. I suppose too much pressure might overpress the joint. I bet pressure was not over 150PSI.

[glenn bradley]

I have starved a joint before. Too little glue and too much clamping pressure on hard maple. You can do it but, if you apply your glue as recommended (a think enough layer that you can't see through it), you would have a tough time squeezing enough out to reach the point of joint starvation.

[Mike Henderson]

No, you essentially cannot over press a joint with the clamps we woodworker's have. Just for a test, I tried gluing some small pieces of pine together and really cranked down on the joint with those steel I-bar clamps. After allowing the glue to cure, the joints were stronger than the wood.

I've never seen, or had a friend report, a joint that was "glue starved".

Mike

[Alan Schwabacher]

I think the idea originates from epoxy use. When you use the thinner (not very viscous) boatbuilding epoxy for gluing wood, letting as much soak in as possible, then mixing in a thickener, applying and clamping is the standard way to go. Epoxy does not grab the way PVA does, so if you squeeze out glue with a clamp, then release pressure, it will likely come apart and not go together again well. Since epoxy does a better job of making a strong joint while filling gaps than do most glues, it can be strong as long as you don't let it drip out of the joint. Leaving an epoxy-glued joint clamped up less tightly just gives you a bit more wiggle room on rearranging the joint. Tight clamping does not hurt otherwise even with epoxy.

[Lee Schierer]

As mentioned by Sam, the process is called sizing and you use thinned PVA type glue. It is primarily used for end grain. Some end grain can suck the glue out of a joint and create a weak joint. If your want to run a test, cut four short pieces of red oak with miters on one end. Glue two of those pieces together at 90 degrees with the normal amount of glue on one surface as most people normally do. Then glue the remaining two pieces together with a first coat of 50% water and 50% glue from the same bottle. Let that first coat dry for a minute or two then glue the joint the same as the first one. Wait 24 hours or more and see which one breaks easier.

[Wayne Lomman]

Overlapping is to be avoided simply because it is unnecessary work. You clamp tight but as said above clamps have short handles because that is all they need. If you feel like you need to add a 3 foot pipe for tightening, something else is wrong.

Letting the glue half dry is wrong. Most halfway good glues including TBII are irreversible chemical reactions. If you recoat when the reaction is half done, you actually reduce the integrity of the joint. Joint starving only happens if you are a glue miser. Following the TBII instructions and applying to both surfaces solves the problem.

The technique sounds a bit like spray painting where you can use the wet on wet method to get a better build in one coat. You apply a coat, let it tack off a bit and then spray a second coat immediately. HOWEVER, this technique is based on solvent evaporation, not curing mechanism. Cheers

[Matt Schenk]

None of my wood-backed bows have ever failed due to the wood-to-wood glue joint. Those were all done with TB III spread on both the pieces of wood and clamped with as many "F" clamps as I could get on.

Here's one of my maple backed cherry bows glued up as described above: https://goo.gl/photos/pBzMRh1rESaeakhB7

[John C Cox]

The advice to avoid overclamping is more for industrial customers who use hydraulic clamping.... You wont be able to overclamp titebond with hand clamps without crushing the wood to pieces.

One piece of advice from the airplane guys.... The best way to ensure you have glue somewhere is to put it there. That means put glue on both surfaces.. And dont skimp - make sure there is squeeze out.... Glue squeeze out is a small price to pay for a good joint.

Another critical item the wooden propeller guys figured out was that planed joints hold up a lot better than sanded joints. As much as possible - joint with a plane or a jointer... Ensure the joint candles well - no gaps.... Use plenty of glue and plenty of clamps.