Today I was frustrated to learn that the glued-up laminations I clamped overnight have a very unreliable glue line, and was wondering if you could critique the following. This hasn't happened before, and I can't figure out what I did differently/wrong.
I glue up straight and bent laminations with powdered PPR glue (in this case Dural brand marine glue). My logic is that a stiff glue line won't creep and show gaps over time like pva might if there is internal stress in the wood. Is this necessary? Should I just use pva glue, or consider polyurethane glue?
When working with PPR for laminations, I mix glue according to instructions on can, let it slake for about 15 minutes. Then, I do a double spread with a short nap roller, getting an evenly glistening surface. I work as fast as possible, usually no more than 15 minutes. Once clamped (in this case pipe clamps both sides, every six or so inches, clamping 8/4 cherry, five pieces, at four inches wide, five feet long), I make a tent out of poly and set an electric heater with a fan inside to crank the heat. (I've wondered if this is a good idea to blow warm air at the wood for so long)I leave it for minimum six hours.
I should mention that the glue was new. I use a card scraper and lightly scrape the jointed surfaces to get rid of tracks from nicks on my jointer/planer. The wood tested at 6-7% mc. The wood came into my shop cold, was machined over about two hours, and then glued up. My shop fluctuates in temp in the winter - thus the heating and hoarding. Some of the cut-offs I tested refused to break at the glue line, while others nearly fell apart in my hands. All the joints had a fairly large bead of squeeze out.
Does anyone have any ideas? Maybe the wood was still cold, and insulated itself from the heater? Could it be a bogus batch of glue? I'm stumped, and now gun-shy to use this stuff again.
From contributor R:
I am not sure why the glue line didn't work, though going from cold board to heating it in a tent for six hours can't help. The wood would move a lot. I look at your process and would suggest you just go to PUR. It is faster, easier, and some of the brands have almost eliminated the foaming.
If the air temps in the shop are down around 50 degrees (or lower), I think you'd be pushing your luck with almost any type of glue. I keep my shop thermostat set to 55F even when I'm not there, because otherwise it takes too long to warm up the materials to the point that glues and finishes work the way they're supposed to.
You said that some of the joints seemed solid while others nearly fell apart. Did you notice any pattern in which fell apart? Were the ones towards the middle of the glue-up more or less fragile than those towards the outside?
As for the use of the card scraper, I should have elaborated: I use it to get rid of ridges from nicked knives, and to reduce/eliminate the corduroy texture left by the planer etc. I may be wrong, but I was taught that a smooth planed surface is better for gluing than a sanded surface because it has fewer torn fibers, and creates better specific adhesion as opposed to mechanical attachment. I would be curious to see if there's a consensus on sanded vs. planed glue surfaces.
After reading the archived topic “getting a good glue joint on cherry” (nearly identical failure) and conducting a water drop test, it was conclusive that the sanded surface absorbed much faster than a jointed surface, and both absorbed faster than a scraped surface. It seems the card scraper leaves the surface too burnished for proper glue absorption. I believe it was a combination of poor temperature control, and inappropriate surface prep.