Wood Glue - Overview
Wood glues are a particularly strong preparation of the common PVA glue used throughout classrooms to sticky up clothes and generally make a mess. PVA Glue (Polyvinyl acetate) is a rubbery polymer which releases water during the curing process, forming a bond whose strength relies on the impregnation of the glue into each surface. Therefore Wood Glue relies on porous surfaces (not just wood) to give a good bond.
Wood Glue typically is available in a range of different colors to match the surfaces, and comes in interior and exterior formulations.
What jobs is it good for?
Wood glue is temperature resistant and water resistant, although with water it tends not to endure long periods of submersion. Any two porous surfaces, even of different types, can be bonded using wood glue - it doesn't need to be just wood.
Not to overstate the obvious, but this makes it the go-to glue in carpentry, and in some cases even boat building (although check the label in that situation as some formulations resist salt water better than others.)
What jobs is it not good?
There's a lot of claims around wood glue about it being better than nails; that needs to be taken with a pinch of salt. Although it can provide a strong bond (in the range of 3,000 PSI to 4,000 PSI), it's prone to cracking in applications which have sharp contact shock or significant shearing stress. If you have a teenager who likes slamming doors for example, wood glue probably won't survive many arguments. No more nails? No, actually, yes nails, and yes doubling up with wood glue...
Although some formulations claim to be temperature resistant, non are high temperature resistant. Room temperature is find, but take care in applications close to heat sources such as kitchen drawers below cookers and the like.
Be wary of using wood glue on tropical hardwoods. These can (depending on age and treatment) produce their own oils. Those oils can interact with the wood glue and reduce it's strength, and can stop the penetration of the glue into the surface. If the job involves hardwoods, apply a thinner to remove those oils first, then soap and water to remove the thinner.
Finally, wood glue isn't good in the presence of solvents, so if the finished article is likely to come into contact with strong cleaning solvents such as acetone, methyl ethyl ketone, toluene, nPB, and trichloroethylene (TCE), then wood glue is inappropriate. More common mild solvents such as isopropyl alcohol, glycerin, and propylene glycol should be fine during reasonable use though (as in, spray on, wipe off situations.)
How to use
This is a very simple adhesive to use. Usually it comes in a tube, and you squirt it out. The usual guidelines apply of course, but this is essentially strong play-glue and shouldn't give many pratical problems.
The thing to bear in mind is the curing time. This will take upwards of 12 to 24-hours to cure fully and give the full bond. In order to make sure that the two surfaces are held together during this period it's common to use a clamp that can apply pressure throughout that period. If that's impractical nail or tacks are often used to apply that clamping stress - the nails aren't there to provide the end strength of the bond, they're just there to hold the two surfaces together while the wood glue bonds.
- Keep both surfaces dry, and if possible keep the humidity low. Wood glue hardens through a drying process, and the more moisture available in either the gradient between the surface and the air, or on the surface itself, the longer it'll take to cure.
- Similarly, adding modest heat will speed up the curing process. Don't go overboard with this one, but the heat from say a hairdryer can help speed up curing considerably (if you don't mind the boredom of holding a hairdryer to a pair of planks.)
- As always, ventilate well, but in the case of wood glues, ventilation (for the same reasons as the last two tips), can speed up curing as well as removing the fumes from the glue itself.
- Make sure the surfaces are clean surfaces. Dust and debris will not only come away as it's not part of the larger structure, but will also stop the impregnation of the wood glue into the surfaces. Moreover, oils and grease can attack the bond. All of these contaminants can be easily removed with soap and water.
- Don't use too much! More glue just leads to a longer curing time, and because the surfaces end up slightly further away can actually result in a weaker bond.