The rights and wrongs of wood glue!

[scrimper]

For many years I have been making my own boards by edge gluing then running them through the thicknesser, I have always used a PVA adhesive to join them. Now in the distance past I used to leave the edges to be glued a little rough (using a TCT sawblade) on the understanding that roughened surfaces made stronger joints, then some years ago my process changed after I read an article (in a book or Magazine, can't remember which or where) to the effect that the smoother you could get the surfaces to be joined the better and that the strength of the joint was at a molecular level.

The process explained was that even surfaces that look and feel really smooth would under a microscope would not be and that what the adhesive did was smooth the surfaces even more enabling them to be bonded at an atomic level, an analogy was given of how two sheets of glass with water trapped between them is difficult to pull apart.

So for about the last 20 plus years I have always made my edges to be bonded as smooth as possible rather than rough and I must say I have never had a board come apart. However tonight I watched a video on youtube where they suggested that you get a much stronger bond if the edges to be joined are roughened!

So which is correct as regards bonding timber is smooth better than rough? Have I been gluing up wrongly over the last 20 + years? (I can see why some adhesives between some materials will bond better if the surfaces to be joined are rough.)

 

[Tetsuaiga]

My understanding is yellow wood glue absorbs very slightly into both surfaces and are also connected by the glue. The glue cures solid and now both all three parts, wood A + glue + wood B are effectively one solid bonded mass.

So toughening a surfaces gives no benefit, maybe some glues need some roughness like varnishes do.

 

[Shrubby]

Scrimper - I believe you're right, PVA works best with a smooth surface. it's epoxies that require a keyed substrate to bond. Much of the stuff on youtube is ferkinshyte
Matt

 

[Phil Pascoe]

And while we're traversing this particular minefield do we put glue on one side or both? Most manufacturers specify only one.

 

[ColeyS1]

I notice westward building supplies are doing 25kg of cascamite for 49.99 + vat. For that price you could put it on both surfaces

Coley

 

[Racers]

One YouTube video won't change my mind. Wood glue isn't a gap filler so smooth clean surfaces I believe are best.

Pete

 

[ED65]

However tonight I watched a video on youtube where they suggested that you get a much stronger bond if the edges to be joined are roughened!

What glue were they using? If it was any PVA type then the YouTuber in question is ill-informed. If they were talking about epoxy on the other hand then that's completely different.

So which is correct as regards bonding timber is smooth better than rough? Have I been gluing up wrongly over the last 20 + years? (I can see why some adhesives between some materials will bond better if the surfaces to be joined are rough.)

With most glues it is the smoother the better for a great glue bond in wood, the article you read was right on the money.

How the smoothness is achieved is important though, a smoothly-planed surface is about perfect but you don't want to try to get the equivalent by sanding to 600 grit or something daft like that because you risk burnishing the surface. Apparently sanding to 320 grit is more than sufficient if you do need to sand.

 

[Droogs]

From what I've learned the condition of a surface to be glued is really dependent on the following factors:
The first being the type of glue available and whether it achieves adhesion by mainly chemical reaction or mechanical means:
if by chemical then a smooth surface is best
if by mechanical then a slightly roughened surface is best
The second is whether the two surfaces to be joined are porous or not and whether they will react chemically with the glue or not. Surfaces that do not react chemically need to have a glue that works mechanically by penetrating a porous surface or permeating a pre-keyed surface (scratches) and visa versa.
It must be said that in most cases all glues and surfaces have both a chemical and mechanical interaction with glue, it is just the proportionality of each within the bond that differs.

 

[MattRoberts]

As with most things, it's about getting a balance. Consider the extremes:

Having two super smooth (1000 grit plus) surfaces clamped together would squeeze out most of the glue, and only have a very small amount bonding the wood.

To the other extreme, having two very rough sawn wood surfaces would mean that the high ridges in the surface would stop the low valleys from connecting well.

The trick is to get the middle ground, and have a reasonable amount of glue with Good contact across the whole surface

 

[scrimper]

Many thanks for all the interesting replies, I have tried quite a few different brands of PVA and TBH they all seem to work about the same, for the last 10 years I have been using the Everbuild brands bought via Toolstation. at the moment I am using Everbuild 526 (D2) (interior use only) it does have slight gap filling properties over the 502 (D3) (Exterior & interior) version both seem to work equally well, I have also used the Everbuild D4 premium adhesive but have not found much advantage over the other two but I did find that the D4 did not seem to have such a long shelf life and I had to chuck the last D4 I had as it went all 'rubbery'.

I have to say that I am confused that Everbuild state spread thinly over both surfaces as surely putting a thicker coating on one surface would give the same effect? When gluing up say 8 strips it is much easier to line them up and spread the adhesive over the one edge in one go than have to do each side.

As for getting the edges square and smooth I run them through the thicknesser but of course although they look smooth it does leave a slight rippled finish plus it is some times difficult to get the edges 100% square so what I do on edges less than 3/4" thick is run them against a 1/2 straight router cutter in a table with a strip of Formica to offset the cut edge, this does result in a nice straight square edge to join.

Obviously no circular cutter will give as smooth an edge as a decent jointer hand plane but that method would take be too long and I doubt I would get the work truly square!

 

[RogerP]

..............
So for about the last 20 plus years I have always made my edges to be bonded as smooth as possible rather than rough and I must say I have never had a board come apart. However tonight I watched a video on youtube where they suggested that you get a much stronger bond if the edges to be joined are roughened!

So which is correct as regards bonding timber is smooth better than rough? Have I been gluing up wrongly over the last 20 + years? (I can see why some adhesives between some materials will bond better if the surfaces to be joined are rough.)

Straight from the plane, there's no need for abrasives when edge joining boards, you're likely to cause problems of fit by sanding (dips, edge rounding etc).

 

[scrimper]

Straight from the plane, there's no need for abrasives when edge joining boards, you're likely to cause problems of fit by sanding (dips, edge rounding etc).

just to confirm I have never sanded the edges to be joined, they are always straight from the plane or router.

 

[Phil Pascoe]

526? I've never seen that one. Is it totally soluble when set, do you know? It would be good for paper joints - the interior Resin W is a pig to clean off, it's half way to being resistant. I think as technology moves on water resistant is becoming the default position.

 

[scrimper]

526? I've never seen that one. Is it totally soluble when set, do you know? It would be good for paper joints - the interior Resin W is a pig to clean off, it's half way to being resistant. I think as technology moves on water resistant is becoming the default position.

http://www.everbuild.co.uk/adhesives/wo ... d-adhesive I used to but it from Toolstation but seem to have de-listed it now.

 

[EddyCurrent]

Straight from the plane is my method. With regard to the glue though, my choice for many years was PVA but I found it would show a slightly raised glue line however much I sanded. Now I use Titebond and it's the best wood glue ever, you won't go back to PVA after using it.

 

[Phil Pascoe]

Won't go back to PVA? Titebond is PVA.

 

[ED65]

Having two super smooth ... surfaces clamped together would squeeze out most of the glue, and only have a very small amount bonding the wood.

But that's actually a good thing. It's sometimes said that the goal is to have a single molecule of glue between the two surfaces, that's an exaggeration but it's certainly true that a glue line can be almost unbelievably thin and still amazingly strong. According to the technical sources (including one or two of the glue manufacturers) it's nearly the thinner the better, with many adhesives a glue line of 0.06mm is weaker than one of 0.02mm.

 

[ED65]

I have to say that I am confused that Everbuild state spread thinly over both surfaces as surely putting a thicker coating on one surface would give the same effect?

In theory it's the same, but applying to both surfaces helps ensure that both are fully wetted with the glue.

 

[MattRoberts]

Having two super smooth ... surfaces clamped together would squeeze out most of the glue, and only have a very small amount bonding the wood.

But that's actually a good thing. It's sometimes said that the goal is to have a single molecule of glue between the two surfaces, that's an exaggeration but it's certainly true that a glue line can be almost unbelievably thin and still amazingly strong. According to the technical sources (including one or two of the glue manufacturers) it's nearly the thinner the better, with many adhesives a glue line of 0.06mm is weaker than one of 0.02mm.

Whilst not exactly the most scientific of experiments, check out Mathias' test here https://youtu.be/14Mkc63EpMQ

The thinnest layer of glue joints were actually the weakest in practice

 

[Phil Pascoe]

I have to say that I am confused that Everbuild state spread thinly over both surfaces as surely putting a thicker coating on one surface would give the same effect?

In theory it's the same, but applying to both surfaces helps ensure that both are fully wetted with the glue.

Many manufacturers actually specify only apply glue to one side only. They obviously have a reason, but I've asked numerous times before and never got a definitive answer from anyone. I cannot see any reason for this theory, personally.