Wetting Wood Joints Before Gluing

Dampening a joint before gluing helps the adhesion of polyurethane glues, but not yellow glues. January 22, 2006

Gorilla glue suggests wetting surface of material of hardwood with a little water before glue up. It seems like it would be a good idea to wet mating materials when using regular yellow type glue. Any feedback? It seems like it would open the grain up to provide a better bond.

Forum Responses
(Architectural Woodworking Forum)
From contributor R:
As a young apprentice over 15 years ago, I was taught to size the glue joint before gluing. Sizing means to put a small amount of glue on the joint and rub it in, let dry for a minute or two and put more glue on the joint before joining. Your comment of getting the joint wet before gluing is correct. I think the glue wetness serves the same purpose.

From contributor D:
Moisture supposedly helps poly glues (Gorilla) adhere and cure better. Regular yellow glues don't need moisture added. Some even say excess moisture content adversely affects adhesion.

From contributor B:
Moisture aids in the curing of polyurethane glues, and from what I've read, makes for a stronger joint. We mist water on one side of the joint whenever we use it. Water, however, dilutes yellow glues and will result in a weaker bond. If you saturate the wood with water, then the glue that penetrates will be thinned by that water.

Sizing the joints with yellow (aliphatic resin) glues is not a bad idea, and we do that when we need a temporary hold on a butt joint glue up. I say "temporary" because we would not let a joint go out of the shop that way. We have a rear insert disk system we use on some joints, but they have to be glued together first. That is when we use a straight butt end joint, but just until the disk is inserted and spanning the joint in the back face of the material. Sizing these joints has significantly strengthened the joint prior to disk insertion.

From contributor C:
The polyurethane glues are described as "moisture reactive" glues, so one has to wet the mating surfaces for a good bond. A customer of mine told me he glued partially air dried pressure treated with Excel polyurethane glue, and didn't wet the surface, but because of the high MC of treated, the bond has held firm since last summer. The project was curved handrail.

From contributor S:
1K polyurethanes cure by using moisture in the atmosphere and the substrate. There should be enough moisture in the timber if the MC is 16-18%. Any lower and you should mist the surface with water. Those sprayers used for misting houseplants are ideal. I have bonded coil-coated steel to Doug fir plywood with these adhesives and they are excellent, but watch out for foaming of the adhesive out of joints if you use too much.

From contributor C:
Do you regularly use timber at that high MC?

From contributor S:
This is the recommended MC for exterior joinery in the UK and western Europe. We often get cabinetmakers producing a few windows for a customer and falling foul because of the moisture content being too low. They manufacture at 8-10%, the timber picks up moisture to equilibrate at 16-17% and the opening lights don't open any more! 1K polys are also used for bonding the various materials in the panels of refrigerated truck containers, usually steel sheet externally, expanded polystyrene core and aluminum or plywood internally. The whole sandwich is smeared with 1K, misted with water, then put in a giant press to keep it flat while the adhesive cures.