Glue joint mechanics - your opinions.

[ByronBlack]

I've always thought and I'm sure i've read more than once and have been told many times that the surface of the wood in a glue join should be smooth, there is no need to 'rough' the surface to give the glue adhesion (where talking about something like titebond here).

Is this correct? Or should the surface be slightly rough for a better join? I think I'm correct - I've always made glue-surfaces smooth and so far have not had any major problems, but someone on another board has taken umbridge with me for having this view - what are your opinions/experiences?

Is there a source somewhere that speficially states or proves one way or the other?

 

[Digit]

As I understand the theory the glue migrates into the pores of the wood and acts like an anchor. Oily woods have the pores blocked with the oil and prevent this migration.
Anyone any other ideas.

Roy.

 

[Danny]

Interesting question, From the little bit of woodwork I did at school, I was always taught to scuff glue joint's up a bit with something like 40 grit sandpaper. I can't really think how a smoother joint would be any better really, but I'm no expert.

Danny

 

[p111dom]

Likewise with me Colin, I've never had a problem with a smooth joint although I do sometimes use a glue joint router bit to extend the gluing surface if I need the strength. It's not like you're spraying paint on to a metal surface here. Wood is a natural porous product. I would have assumed to a certain extent the glue would soak into the wood ever so slightly which would have the same effect as roughing an edge up. The straighter the edge the better looking the joint. Tell him he's talking rubbish. Go on light the blue touch paper and run! :lol:

 

[Chris Knight]

Byron,
I have done loads of tests with glues. The idea of a rough surface comes from generally ill-informed notions of a mechanical key being important in a glue job whereas it is a molecular bond that is really important and this is at a scale far, far smaller than any mechanical key. Mechanical keys can be useful with some glues, notably epoxies, where the glue itself is strong and capable of filling gaps.

A smooth surface is generally better than a rough one which can have loose or weak fibres that reduce joint strength.

Having said this, it is important to ensure that surfaces to be glued are free from contaminants and a good way to do this is to lightly sand with 100 grit paper just before gluing. There is enough oil from your fingers to compromise glue bonding so the sanding is very important if you have been handling a piece like a rail with tenons at each end a lot.

 

[woodbloke]

Byron,
I have done loads of tests with glues. The idea of a rough surface comes from generally ill-informed notions of a mechanical key being important in a glue job whereas it is a molecular bond that is really important and this is at a scale far, far smaller than any mechanical key. Mechanical keys can be useful with some glues, notably epoxies, where the glue itself is strong and capable of filling gaps.

A smooth surface is generally better than a rough one which can have loose or weak fibres that reduce joint strength.

Having said this, it is important to ensure that surfaces to be glued are free from contaminants and a good way to do this is to lightly sand with 100 grit paper just before gluing. There is enough oil from your fingers to compromise glue bonding so the sanding is very important if you have been handling a piece like a rail with tenons at each end a lot.

Byron - I have to agree entirely with Chris here. The only thing I would add is that instead of lightly sanding with 100g paper a quick scrub over with some acetone on a stiff brush will also remove surface grease and is an essentilal part of the gluing process with an oily timber like teak - Rob

 

[stuartpaul]

I'm a 'smooth joint' man and always have been (thank you Sir, - you taught me well!!).

Does never having a joint fail work as an argument? I'd never even consider a 'rough' join unless it was for very crude work and appearance didn't matter.

But then again .....

 

[beech1948]

BB,
I was taught some 46 years ago that the glue line should be smooth and finished with a plane and not sandpaper.

I was also taught that glue lines should have the maximum of pressure applied as a thin glue line is much stronger than a thick one.

regards

alan

 

[Pecker]

certainly I was taught to use smooth glue lines. I have been known to occasionally rough up an oily timber with fine sand paper, but otherwise for the past 30 years it's been smooth.

woody

 

[eggflan]

Smooth for me
Unless it is a rough arsed coffee table that i have been known to make, and for that its 2" timbers so i think there is enough wood to get a good bond :lol:

This has stood me in good stead for the last 17 years so i aint changing a thing
Mic

 

[Losos]

Mechanical keys can be useful with some glues, notably epoxies, where the glue itself is strong and capable of filling gaps.

BB - I'm no expert but i have some experience of epoxies and as Chris has said the usual advice with them is to roughen the surface a little, using (as has also been said) sandpaper. With epoxies it's more important on metals than on wood since there is far less absorbtion into the substrate on metals than with wood.

As mentioned in the case of wood, regardless of the adhesive being used the most important thing is to remove oil and other contaminants.

 

[motownmartin]

I am no expert and I don't really know the answer but I would agree with peeps that say a smooth surface and the reason why would be, if you were gluing boards together for a table top and you roughed the edges, I think you would be a lot more likely to see the join.

Just my 2 penneth worth

 

[Danny]

:shock: Proves what my woodwork teacher knew then. I've often roughed up joints thinking I'm doing a better job and achieving a stronger joint. :x Ah well, at least I'll be saving a bit of time now.

 

[MrJay]

I'm lead to believe (and I have no expertise, this is my backside doing the talking) that it all depends. I think what you're after is contact between glue and freshly/cleanly/crisply cut wood fibres. A cut from a good sharp hand tool will do that, or a fresh sheet of coarsish sand paper will do that for you too. What you don't want so much is the kind of rubbed/buffed smooth surface you achieve by way of fine sand paper, dull saw or power tool

I wouldn't quote my backside publicly - it's been known to be wrong.

I have spent a little time in communication, while building my boat, with SP/Gruit who make epoxies, they've got some exhaustive technical info and expertise available. I reckon it's worth a phone call or firing off an email to the manufacturer for some proper technical support if you've got a proper technical question. I suspect the directions on the back of the tin are pretty generic.

 

[ByronBlack]

Thanks to everyone for their feedback on this, it's pleasing to know that I wasn't talking out of my rear-end and making a fool of myself! I've since carried out a little more research on the 'net and as digit has said - the glue 'hooks' into the pores and creates the bond, so a smooth open-pored surface is ideal, and apparently the sanding pushes and 'mushes' fibers into these pores causing less penetration and a weaker bond!

So, thanks again!

 

[colinc]

A subject close to my heart - I'm building a wooden aircraft, so reliable glue joints are essential.

Aircraft practice is that wood to wood joints are not sanded or roughened, the shorter the time from planer to glue the better as the fibres are cleanest that way. Older wood is best cleaned with a plane, scraper or a similar tool to remove shavings which can be cleared away easily. Sanding leaves loose abrasive residue from the paper and fine dust which is difficult to clear away from the wood pores - both can interfere with good adhesion. Where sanding is essential on things like certain laminated parts then cleaning with a dampened cloth helps pick up the dust.

Conversely, ply is always sanded to remove the waxy surface residue from the pressing process, again a damp cloth helps pick up the dust.

At this time of year temperatures are also an inhibiting factor for some glues. I am using Aerodux, a resourcinol-formaldehyde 2-part resin, which is temperature critical but otherwise almost foolfproof.

I have to make and keep test pieces and a log for every glue mix and glue-up I do. I make two samples. One is for me to bust with a big hammer to check that the wood fails, not the glue line. The other I'll keep for long-term reliabilty testing (or for the accident investigators!!).

regards,

Colin

 

[Digit]

How about some pictures Colin?

Roy.

 

[dickm]

A subject close to my heart - I'm building a wooden aircraft, so reliable glue joints are essential.
[...]
I have to make and keep test pieces and a log for every glue mix and glue-up I do. I make two samples. One is for me to bust with a big hammer to check that the wood fails, not the glue line. The other I'll keep for long-term reliabilty testing (or for the accident investigators!!).

Now that's really putting your a**e on the line. Or worse.

 

[colinc]

Now that's really putting your a**e on the line. Or worse

I hope not. Wooden aircraft are very long lasting and have been around since the first days of flight. There's nothing complicated in building them, just careful choice of material, good glue and patience.

I'll post some picture soon.

regards

Colin

 

[head clansman]

Hi Guys

When i started my apprenticeship back in 1966 the very first thing i was taught was plane the joint straight and square so the two edges touch in there entire length and wow betide me if i ever picked up a piece of sand paper the strength is in the finish smooth planed edges from a hand plan, not a planer joint-er with both edges spread with cascamite glue mixed to a creamy finish join together both edged with as much pressure as you can get from the cramps wiping away all excess glue there is no other way .