I'm working on a project that involves butt jointing a 3/4 plywood edge to an unfinished plywood face. I'm using 18ga brads and want to fortify with glue. Not interested in 2 part epoxies. Is polyurethane the best choice for this?
[OP]
Contributor
I'm working on a project that involves butt jointing a 3/4 plywood edge to an unfinished plywood face. I'm using 18ga brads and want to fortify with glue. Not interested in 2 part epoxies. Is polyurethane the best choice for this?
Scott Vroom
I started with absolutely nothing. Now, thanks to years of hard work, careful planning, and perseverance, I find I still have most of it left.
Contributor
You said unfinished plywood so yellow glue will work fine.
John
Contributor
I'd also consider pre-drilling and then screwing it together, instead of nails. That works better for me.
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Member
NOT wanting to start a "biscuits don't add any strength" argument, but that is what I would use. Any glue will glue that top layer which could rip off with the slightest bit of load. I guess to avoid the biscuit discussion. one could use dowels, but dowels require a bit more precision. Assuming you want to avoid metal screws, which is my first choice.
Just my $0.02.
Contributor
Glue won't be much help if it's subject to any torque at all. Yellow glue is as good as any, it will be much stronger than the wood. If your piece has to have any structural strength then I'd definitely set the ply into a groove (with or without one or two shoulders) or use a spline. Biscuits, dowels. or dominos would also be an improvement, though more work and not as strong. Screws would be stronger than small nails.
You don't indicate whether the joint is edge to edge or edge to face to make a corner. In edge to edge joints you will get an extremely weak joint and I would suggest making a lap joint. In edge to face joint to make a corner your best bet would be to reinforce the joint with wood screws and if possible a wood gusset the full length of the indie corner. Either regular wood glue or polyurethane glue will give you a strong joint. If you elect to use polyurethane glue, do not wet the surfaces. All the wetting does is to promote excess foaming. There is enough atmospheric and wood moisture to cure the glue and you will get a stronger joint.
Lee Schierer
USNA '71
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Contributor
Biscuits excel in plywood butt joints & they absolutely do add a lot of strength.Originally Posted by Kyle Iwamoto
Contributor
Absolutely. I've made lots of cabinets with both plywood and Melamine with nothing more than glue and biscuits. Never has a single joint come loose or broken.
John
Moderator
This. And PVA is good glue for this purpose.
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Contributor
I have never had a biscuit joint fail. They are a good choice for plywood. I have seen lots of dowel failure in commercial furniture.
Charlie Jones
Contributor
Agree, sometimes biscuits are just a little tight right out of bag, then if you just give them a quick scuff on sandpaper ,
they work fine.
Contributor
Generally speaking a butt joint isn't a strong joint without reinforcement with screws, dowels, or some other method like biscuits or Dominos. The type of wood really isn't the main issue. It's the fact that you are gluing end grain to edge grain. With plywood it isn't exactly the same type of surface but the edge of a piece of plywood definitely wouldn't hold as well even against the face of the plywood.
Member
I agree, what caused the “biscuits add no strength” thing is some article’s talking about how biscuits are useful for alignment. Many woodworkers made the assumption that they were just for alignment. They are terrible for alignment but very good for strength especially if you fill up the slots with glue.
Member
If the plywood actually has alternating grain directions, then some of the veneers are actually edge grain and some are end grain. I don't think they'd add much strength. I usually put in a couple of brads and then pocket hole screw them together (most of my cabinets are this way. I just make sure that the pocket holes are on the back, inside or bottom of whatever I'm making. I do use PVA glue with them.
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