Q. I have been making furniture for years and recently had an order for some outdoor pine furniture. I used red pine as it was available. Glued up a few things for outside using a PVA that is suppose to work in exterior conditions. It was just fine until the first time it got wet. Not all of it but most of it came apart. Some of it held good, some you could pull it apart, and some just came apart before I pulled on it. This was the same pile of lumber I used for inside furniture, dried to about 8 percent MC, same conditions in the shop. What is going on?
A. I am assuming that the wood was kiln-dried using temperatures over 150 F to under 12 percent MC (if not kiln-dried this way, we would have some resin issues) and that you used the correct pressure, temperature, etc. (For example, too much pressure with pine and you will squeeze out all the adhesive and end up with a starved, weak joint.)
Was the joint surface prepared minutes before you glued it? A delay between surface preparation and gluing can lead to a weak joint. It is not necessary to wipe the wood surfaces with acetone prior to gluing pine.
Due to wetting and drying and the resultant large movement, the joint strength required for exterior exposure is much larger than for indoor stuff. So, although you had a joint that was OK for inside, the large swelling and shrinkage forces broke the joint outside. What is needed is a stronger joint, and that is why it needs immediate gluing, very flat surfaces, etc.
Within each growth ring with red pine, southern pine and a few others is a large band of denser, darker colored wood (sometimes called latewood). Due to the high density, it is quite difficult for most adhesives to form a strong bond with this wood. So, even the best bond with pine will not be as strong as a wood such as eastern white pine (small latewood bands) or birch.
An exterior PVA should be fine. Actually, for exterior furniture, I would tend to prefer a UV-stabilized epoxy (but remember that epoxy needs a thick joint so avoid much pressure) or a resorcinol adhesive.
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