How should wood surfaces be prepared for gluing?
When you apply paint, the surface is supposed to be clean, dry–and rough, so the paint gets a better grip. Does the same hold true for wood, i.e., should a smooth face be roughed up (with coarse sandpaper, a Sur-Form rasp, etc.)?
Occasionally, somebody mentions a glue-up that failed because of “starving the joint.” Is this more of a problem with wood that is fine-grained, with few open pores to hold glue?
If I first coat the surface with water (perhaps with soap added), will that help the glue penetrate the fibers?
Janet
Replies
Dw,
There are a couple of ways that work. One is straight off the tablesaw provided that you are using a quality blade, your cut is straight, and there are no gaps between the mating surfaces. This is not hard to do with shorter pieces or if you ar lucky enough to have a straitline rip saw. This is usually only found in larger shops. The second is to take a couple of shavings off with a jointer plane. This will straighten the edge and make it glass smooth. That is the best surface. The power jointer is not the best way as it leaves a series of hills and valleys and if the knives are not freshly sharpened it can bludgeon instead of shearing the wood. This does not result in a good glue surface.
As to the question regarding a starved glue joint, you should use enough glue so that little beads ooze out of the joint. It shouldn't run down all over the place although that won't hurt it but it does make a mess and it won't make the joint any stronger. If you are not seeing some squeeze out then you are probably starving the joint.
Mark
Glues bond at the molecular level so definitely do not add soap. It's best to coat the surfaces twice with glue than once with water and once with glue if the glue is tacking up too quickly. I'll assume the glue we're talking about is PVA.
Burnished surfaces are no good. Burnt surfaces are noo good. Burnished surfaces result from dull cutters, whether from a jointer or a saw or a plane. Surfaces from a good blade, a jointer and the best, a hand plane, are all good. Don't use sandpaper unless you need to open pores. Glue fresh wood, old surfaces are contaminated and will not take glue as well.
Lee
Furniture Carver
Can a joint be clamped too tightly, i.e., so tight that too much glue is squeezed out?
Are glues made to a thick consistency so they'll be less messy? I was wondering if glue would "take" better if it were thinned slightly.
Janet
Theoretically, yes, it can. You could have a hard smooth surface and so much clamping pressure that the glue had nowhere to go but out. That said, you'd probably have to spend some effort to get there. Typical edges with anything mentioned above - a plane, jointer, strait-line, leave enough things shy of perfectly smooth not to sweat it, and unless you're putting clamps every two inches . . .
It's one of those things I come across in books. "Don't do this" - but I've never had it happen. Suspect I'm like most people and like a good deal of clamps holding everything together." Clothes make the man. Naked people have litte or no influence in society" - Mark Twain
Janet,
Been there, done that. I had an occasion recently to take apart a large carcas that was one of the first things I made in the shop. I glued and screwed it orginally. After taking the screws out it fell apart in my hands...I had plenty of glue and too much pressure. The pressure made the piece somewhat cockeyed too....
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