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What would be the best way to glue two surfaces which have already been finished with Danish oil and then wipe-on poly?

One surface would be the cut edge of ply, the other surface would be solid wood. Specifically: a long 2 1/4” width of solid wood capping two other edges of ply separated by a 3/4” gap.

I won't be able to biscuit or dowel, but the good news is that this isn't structural - just a finished surface. Nevertheless, it's not an ideal situation, so some collective wisdom would be helpful here.

Ola Ström
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Dale
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  • Follow-up note: The cut edges of ply still look fairly porous, so, the bigger problem is the solid wood. Perhaps if I just rough-up the solid wood with a file or something??? – Dale Feb 05 '19 at 17:45
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    Honestly, the best way is to remove the finish to bare wood so the surfaces mate properly and glue it up. This is especially true if edge grain is involved at all. (BTW, you should be in the habit of [edit]ing the question text to add details. Comments can be deleted and are not strictly part of the question.) –  Feb 05 '19 at 17:48
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    Some refs: https://woodworking.stackexchange.com/a/2348/5572 https://woodworking.stackexchange.com/a/6941/5572 https://woodworking.stackexchange.com/a/364/5572 (I couldn't find a specific Q&A that addressed the specific context here. This may be an open Q&A for a wiki reference answer.) –  Feb 05 '19 at 17:55
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    No, don't rough up the solid wood. Generally the smoother the surface the better for a strong glue bond (this is why the super-smooth edges a jointer plane can create glue together so very well). For a non-structural thing this isn't that critical, but something to bear in mind for the future. – Graphus Feb 05 '19 at 18:31
  • "What would be the best way to glue two surfaces which have already been finished with Danish oil and then wipe-on poly?" I can't tell from the Q if there's any finish on the edges, if there is it should be removed by sanding or scraping, or planing if you have any planes. After that just glue and clamp, it doesn't have to be any more complicated than that. – Graphus Feb 05 '19 at 18:33
  • Thanks jdv & Graphus. To answer Graphus’ question, both surfaces have the poly. Sounds like if I can clean up the one side, I can get a fairly decent bond with the ply edge, even though the danish oil and poly have penetrated that ply edge. There’s still a lot of porous bits there that should help the glue bond with the ply. – Dale Feb 05 '19 at 18:39
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    "There’s still a lot of porous bits there that should help the glue bond with the ply." You won't normally get any bond worth noting in porous stuff, this is why end grain joints glue weakly. And it's why edge joints in MDF and plywood are noted for not being strong. Most glues are only strong in a very thin film between two surfaces that are in intimate contact. I would recommend you glue with epoxy which is one of the only common adhesives that works well thick (and also does not require strong clamp pressure to achieve strength, which is the case with almost all other glues). – Graphus Feb 05 '19 at 18:57
  • Thanks again, Graphus. I just read something similar referencing epoxy. – Dale Feb 05 '19 at 19:24
  • Following up... The labels on the epoxy state not to glue with polyurethane plastics. Since the wood was finished with polyurethane, that seemed like a bad bet. Ultimately, I had to scrape/sand away as much finish as possible on the solid wood (it’s not edge grain, jdv) and the cut edge of the ply. Not easy to do with the ply, but the solid wood no problem. There’s still a bit of the Danish oil that’s soaked into the wood... I could tell by the nature of the sawdust coming off... clumpy, etc. Anyhow, finally got to gluing with regular wood glue and it’s holding up very well. Thanks again. – Dale Feb 08 '19 at 04:55
  • @Dale if you self-answer you get that sweet reputation boost, and this becomes a bit of a canonical answer for the specific question that I could not strictly find elsewhere here. –  Feb 11 '19 at 14:33

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Short answer: remove the finish, smooth gluing edges, glue, and clamp.

Longer answer: apparently what is being joined are two slightly irregular surfaces that cannot be smoothed ideally. Recommended solution is to remove as much of the finish as possible (scraping should be more than enough removal) and epoxy the joint.