I received this question in episode 5 of the Ask Brian Boggs Show:
What glue is appropriate for bent lamination that will be outdoors? Type 3 PVA, polyurethane glue, epoxy, or something else? Or is steam bending a more reliable alternative for this application?
I have done both. The first outdoor chairs I worked on were actually steam bent back slats in both walnut and white oak (pretty good outdoor woods). The client for this job insisted on domestic wood so the choice was limited.
Those did well but if you are going to steam bend for outdoor use, I strongly recommend kiln drying the parts even if it’s just a box kiln. This will help get your temperature well over 100 degrees and help in getting moisture content well below 4 percent. The point is that you are locking the lignin in place so that you are essentially baking it so that it will take more than rain to straighten these pieces out. You will need to steam them again. I was concerned about the order (mentioned previously) because there was going to be 20 chairs, with 100 slats that have to be holding their curve. I actually performed a test by putting the slats (which were steamed, bent and baked to cure) back into the steamer and could not get them to bend. That does work.
We do all of our bends in outdoor furniture now because we are using mahogany, which is not particularly good bending wood. We use an epoxy that I like from Rot Doctor called All Wood Epoxy Glue. The reason it is so effective is that the resins are made from wood fiber, natural wood resins. They integrate very well with particularly oily woods, like teak or mahogany, and you get a great deal of absorption. This is another reason why you want to let the wood have time to absorb what it needs before pressing parts together. Also, make sure you have not left dry spots where you will have to go back and recoat.
All Woody Epoxy Glue is a great choice for this project. I have tried polyurethane glue with disastrous results. PVA glues are moderately ok but not a great choice for a lamination due to the fact that they do not get hard enough. PVAs remain too elastic, which gives your connections the inability to be stiff enough to bond and also prevents movement of the bonded laminations. I strongly recommend an epoxy like All Wood Epoxy Glue.
During the winter time, we do not like to heat our shops if outside temperatures are at least 45. A lot of epoxies will not work at that temperature. Therefore, another reason I like All Wood Epoxy Glue is that it works down to 28 degrees. For those who do not want to heat your shop during the winter time, All Wood Epoxy Glue is a great choice.