I scoured the interwebs for a while to find out whether wood glue works for including a layer of fiberglass between plies of wood in a laminate, but I didn’t find any answers. So, I ran a small experiment.
Both of these samples here are pieces of maple from Roarockit, laid up in the same way. Both interior faces of wood wetted out with glue, with a layer of fiberglass sandwiched between them that was fully wetted out with glue using a squeegee. Both compressed for 24 hours under a stack of books.
I bent each sample along the grain in a couple different spots see how it broke. The difference in the breaking strength and the crack patterns seems to me like it had more variation between the different regions of grain on each sample, than the degree of variation between the samples. So I would tentatively say that the difference between wood glue and the more traditional epoxy is not very large for this application.
Of course there is. When all you have is a hammer and chisel, every problem looks like a test for resistance to delamination.
Surprising results. The epoxy in question is the composite envisions brand thin laminating epoxy, with the slow hardener. I would characterize it as low viscosity (it soaks into wood very well), and slightly more brittle than the average epoxy. I could repeat the test with Total boat High performance, but TBH the difference in brittleness at full cure is way smaller than what I saw between the epoxy and the wood glue. The difference in strength between composite envisions and total boat might make a difference, I don’t know.
In the epoxy sample, my chisel skated right over the surface of the fiberglass and peeled the upper layer of wood clean off. No tear out from the wood, just a failure of the epoxy.
In the bending test, it was slightly stiffer and slightly more brittle. In the Chisel test, it was slightly harder to begin the peel failure but very easy to continue it.
In the wood glue sample, my chisel was met with much higher resistance, and ended up cutting through the fiberglass accidentally instead of continuing to peel off the upper layer of wood. The wood had some resistance to being broken up from the laminate after the chisel was under it. There was a moderate amount of tear out in the wood as the fiberglasses peeled off of it.
In the bending test, it was slightly mushier and felt a little less stiff than the epoxy, making me wonder if it might be subject to creeping failure over time. In the chisel test, it felt much mushier and had a far greater toughness.
The fiberglass layer in wood glue feels like it has a consistency similar to rubberized canvas, rather than a structural plate like a piece of fiberglass epoxy and feel like. So it really surprised me with its tenacity.
Interesting experiment, but for the fiberglass to add a benefit in your layup, it needs to be on the outer skins, ideally both sides of the core, and adhesion to the surface of the wood core is then the main factor.
That is the most structurally efficient in achieving bending stiffness, but that’s more of a secondary goal for what I’m working on. I’m interested in getting toughness and screw holding strength while leaving the bare wood grain at the surface for the a e s t h e t i c
So I’ll put the glass underneath the face plies, which cuts its effect on the flex in half but will engage a flat head screw more reliably
I’m not surprised at the results to be honest. It is well known that a properly done glue joint ends up stronger than the wood itself and fiberglass is porous enough to be able to absorb the glue well.
Only real downside I can think of is that you will probably end up with a good amount of wasted wood glue to make it but that happens when gluing wood to wood anyway
I soaked the ends in a pot of water for a couple of hours, the first thing to note is that one of the regions on the epoxy sample which was already cracked, delaminated by itself and buckled outwards from the expansion pressure.
The Chisel tests have the same ranking as the first time. The epoxy sample was a bit easier to get started, and still had low resistance to the chisel. The wood glue sample had its toughness much reduced, but it still provided a good deal more resistance than the epoxy sample.
Well I think it probably takes the same volume of glue to fill up the fiberglass in either case, but wood glue is much cheaper by weight so I don’t think that’s a waste in my book
I would just personally use more wood glue than I would when gluing wood to wood so that way you know it is fully saturated. Also me saying that is the only issue means that everything else looks good lol
Yes, I did it the same way I perform a layup with epoxy. Both bonding surfaces of the wood were lightly wetted out, and the fiberglass was wetted out separately on a drop cloth until it was fully saturated, then transferred into the middle of the sandwich.
This is something I’ve given a great deal of thought to in the past.
I’m pretty sure wood glue tends to be more ductile, so one of the things Ive wondered about is if there might be practical improvements to impact resistance somewhere.
Of course, any application is moot if it doesn’t hold up with the lamination stregnth.
Was just thinking of an analog to riding on a wet street or a muddy trail, then festering in a closet overnight.
a e s t h e t i c 
