I’m new to woodworking and would really appreciate a tip.
I need to glue an untreated piece of black walnut to plywood that has been treated with polyurethane stained varnish. Question is – should I sand the polyurethane off before gluing, or leave it. Both surfaces are flat and smooth otherwise.
Many thanks, all.
Replies
You should sand off the poly.
This will give the strongest joint and there is no danger that incompatibility with the glue or the poly being weakly applied will cause problems.
That having been said, if you use polyurethane glue and leave the poly in place, that sucker is NEVER coming off - it will stick almost anything to almost anything, especially if you don't want it to be stuck...
Agree with Rob SS above - also you didn't say what you meant by gluing walnut to plywood... If you mean gluing a strip of walnut on the edge of the plywood that will work fine. If you are gluing a wide piece of walnut to the face of a wide piece of plywood (say 6") that won't work well. The walnut will expand and contract and the plywood will not, so the pieces will pull apart with time and weather changes.
If you give us more details, maybe with photos and/or drawings, we may be more able to better advise.
Thanks both for your replies - I really appreciate it.
The idea was to glue down a wide piece of walnut to the face of a wide piece of plywood - the expansion issue hadn't even crossed my mind...
It's essentially to replace my existing headboard (I've attached a few photos). I'd thought to simply glue it and clamp it but perhaps this was a bit naïve. I could possibly remove the plywood altogether (though this is much more of a hassle for several reasons). If you have any other ideas I'd really appreciate your help.
If you can put a few screws up through the plywood into the walnut, that's how I would attach it. No glue.
Thanks, John_C2. Screws are possible for the part marked in red on this pic - but unfortunately for the other areas.
All right, you can do this... No perfect solution here, but this will work:
- Glue down ONLY along the entire thin strip of walnut on the left in the photo by your pillows*.
- You are going to screw down the section marked in red but you have to allow for expansion and contraction across the grain (ie L to R). SO make slots in the plywood for the screws to attach from underneath - about 1/2-3/4" long and L - R in orientation. Maybe three along the edge where you made the opening and three close to the right edge. Then attach your screws in from underneath in the center of the slots so when the wood expands and contracts it can move.
- Not perfect, but just leave the little strip under the window alone.
* - When you do glue-ups, you need to clamp the parts together until the glue dries. Do you have clamps for that?
Great- appreciate the advice. Only other thing I forgot to mention is that I live in a very stable climate (Singapore) 28-32 deg celcius all year round without much variation basically (daily humidity can vary but average monthly humidity is also very stable all year round). I’ll go with your suggested approach but just wonder if the climate should make me more or less concerned about expansion/ contraction effects?
(… and yes have clamps)
Much less concern, but I still wouldn't risk gluing the whole thing down!
I live in Northern California with few problems with expansion/contraction of wood as well, but still design for it. Ironically it is very dry most of the year here, but my memory of being in Singapore is that it is very humid - so moving any wooden objects between our two sites would be seriously problematic!
Good luck!
I don't have any direct experience with trying to glue polyurethane finished wood with polyurethane glue, but I do know that even polyurethane won't stick well to cured poly. I've seen a whole family room floor delaminate because the finisher didn't get the timing right on the second coat; after 48 hours or so, the first coat was too cured for anything to stick to it. So I would sand off the poly.
You could recess some keyhole fasteners into the underside of the walnut board. Then put screws into the plywood that match.
You would then mount the walnut board with a sliding motion - the same way you mount a power strip on the wall.
Mike
For that application I would scuff the top of that shelf unit or whatever it is and glue the walnut top down with construction adhesive. I'd use something like PL 400 but almost any type will do. Except maybe "liquid nails" which doesn't seem to be much good for anything. I'd run a bead about 3/4" back from the edge all around and then make a bunch of x's and o's through the center, clamp it down or stack a bunch of books on it even. The construction adhesive will hold it down but there is enough flex in it to negate any worries about expansion. Then finish and seal the walnut with whatever you prefer.
As I was reading through this thread I was thinking along the lines of Pantalone’s suggestion. Along the same line of thought you might even get away with running a bead of silicon in the pattern he described. The stuff sticks and flexes surprisingly well and you can easily reverse and be back at square one without any damaged parts if it fails for some reason (which I doubt it would). Place a heavy object on it while it sets up. This is obviously well out of the realm of fine woodworking, but plenty of surfaces are held down this way including the vast majority of kitchen countertops. Good luck however you proceed
This forum post is now archived. Commenting has been disabled