Why glue on both surfaces?

[Andy Haney]

I just read some of the clamping survey, and it brought up a question in my mind. I've forever heard that glue should be applied to both mating surfaces for the best joint. While I've accepted that and do it religiously, I've never known WHY that makes for a better joint. Please someone, explain it to me in elementary terms.

Thanks,
Andy Haney

[mark page]

Hi Andy,

Wood is a porous material. If you put a drop of water on wood it will absorb it. The same holds true for glue. If you do not put glue on both sides/ends, etc, you may end up with a dry glue joint. If this is the case the wood absorbs the glue to where there is none to hold the mating surfaces. End grain gluing is the worst for this to happen such as in miter joints of picture frames, boxes, etc. This is also the same properties as to why it takes more than one coat of finish on completed projects. Hope this is basic enough and helps you out.
Mark

[nick brigg]

im just guessing here...but i use white glue and white glue is water based...by applying glue to both surfaces you are moistening the wood and letting the glue enter the cells of both parts of the wood. so when the glue hardens it becomes a damn good bridge between the cells of both woods.

i could be wrong but hey, makes sense to me at least...

[nick brigg]

hah i see someone got to it before i did.

[Chris Fetting]

This is a myth. Recent study in Wood Magazine concluded that as long as you use the proper amount of glue you do not need to wet both surfaces.

[glenn bradley]

Depending on the wood, the assembly and the time required to get the clamps on I glue one or both sides. If you assemble and clamp quickly and get good squeeze out, gluing one side is OK IMHO. If the wood in general or on end grain is very absorbent, you can get a starved joint. These generally fail or allow problems that would not otherwise occur.

[Mike Henderson]

This is not exactly the same, but I can relate a problem with veneering where you only apply glue to the substrate. When veneering, you can't put an excess of glue because it'll come through the veneer. And if you don't put enough, they'll be places where the glue on the substrate didn't wet the veneer and you can have a bubble.

Extrapolating from that, I would think that if you did not put enough glue on the one side of your glue joint, you could have places where the glue didn't transfer to the second piece - and these places would not be glued together. With major pieces of wood (not like veneer) these gaps wouldn't matter much. When you put glue on both pieces, you guarantee that both pieces are wet with glue and minimize the chances of gaps in the glue.

Again, I doubt that the strength of the joint would be compromised by a few glue gaps but, after working with veneer, I always put glue on both pieces when doing full size joinery.

Mike

[Jules Dominguez]

I've done it both ways. I think putting glue on both pieces is a "belt and suspenders" approach. Sure won't hurt anything unless you put so much on that you get excessive squeeze-out.
If part of the glue line is very porous, I rub some glue on it to seal the pores before doing the glue-up. I think I read a recommendation to that effect somewhere a pretty long time ago. Maybe Tage Frid?

[Ron Brese]

I personally think that you need to apply the right amount of glue in lieu of putting glue on both surfaces. When you apply glue to both surfaces you typically get too much glue and excessive squeeze out is messy, causes finish problems, and waste glue. I think this is an idea perpetuated by the glue manufacturing companies to increase glue usage. I've never had a call back that was glue joint related, and that's alot of glue joints over the years done by applying glue to one surface.

Ron

[Steve Rowe]

Try this;

Put glue on only one surface
Put the pieces together and rub slightly
Pull them apartIs there glue on both surfaces? If the answer is yes then, there is no need to apply glue on both surfaces. If the answer is no then, you didn't apply enough glue or the surfaces are not making full contact.

Steve

[Rob Millard]

Andy,
It really depends on the joint. An edge joint doesn’t have to have both surfaces coated. As long as you apply enough glue to the one edge, your resulting joint will be prefect. On a mortise and tenon joint, I apply glue to both the tenon and the mortise. I do this because the piston like fit of a tenon in its mortise would have a tendency to squeegee the glue off the mating surface and leave the joint insufficiently glued. I would do the same for dovetails in thick material, but for drawers, I’ve found gluing just the pin board is enough. I work almost exclusively with hide glue and I vary the consistency of the glue for different joints. I use a pretty thick glue for edge joints, and a much thinner glue for dovetails and mortise and tenon joints. The thin glue prevents having it pool in the bottom and interfering with the joint being driven fully home.
Rob Millard

[Kelly C. Hanna]

I do what Steve does....exact same result as putting it on both surfaces...I've tried both ways with no difference.

[John Huber]

Here's a reason to apply glue to both surfaces. I often glue dissimilar species of wood. One species may be more absorbent than the other. I apply glue with a brush and apply just enough to both pieces so the surface doesn't look dry after 10-15 seconds. Then clamp. This procedure minimizes squeeze-out, but assures that there is enough glue absorbed into both pieces for a strong joint.

Put another way, if I only apply glue to one piece, how do I know in advance that there was enough glue to be absorbed into the other piece? I'm not comfortable with a "learning by experience" approach.

[Lee Schierer]

This sounds like a myth to me or was an attempt by someone to sell more glue as was stated above.

I've been making glue joints since I was in 7th grade and never had one that I applied glue to both sides. I can also count the number of joint failures I've experienced on projects I've made on one hand not using all the fingers or the thumb. If you apply sufficient glue to one surface, it should apply the right amount to propely wet both sides of the joint. Too much glue creates extra work and may cause finishing problems. Too little of course may create weak joints, the trick is getting the right amount and practice makes perfect.

[Cliff Rohrabacher]

This is a myth. Recent study in Wood Magazine concluded that as long as you use the proper amount of glue you do not need to wet both surfaces.

I agree. So long as you apply enough and so long as either it spreads itself or you do it you should be golden.

The idea that the glue needs to settle in (wet & absorb) into the wood on both sides appears not to contemplate that unless the craftsman is waiting till the glue skins over that it will indeed do it's share of wetting on both sides when the pieces are brought together.