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Thread: "Cheat" to flatten wood

  1. #1

    "Cheat" to flatten wood

    I'm building a countertop out of air dried (5+ years) english walnut. My stock is about 86" long, 8-10" wide and 4/4 thick. My wood rack must have sagged slightly because the boards have a 1/4" bow at a spot about 1/3 of the length. My jointer is only 6", and my planer 12".

    I don't really want to end up with 1/2" thick wood and besides I'm not keen on building an 8' sled for my jointer to flatten the wood.

    Since only one side is going to be visible, and since the top is going to get covered in a 1/8-1/4" coat of clear epoxy resin (preventing exposure to moisture), I'm thinking of just gluing the walnut down to a substrate of mdf that is clamped tight to my workbench. By alternating the boards I can keep the "sag spot" off the same place on the mdf, and hopefully the mdf will keep it flat enough for the epoxy to make up the difference. I tried clamping the walnut to my bench was pretty easily able to clamp out the warp.

    Thanks for the input.

  2. #2
    Join Date
    Dec 2008
    Why not just alternately flip the boards end for end so you have the sags in different locations and dowel the whole thing together referencing off one face. May take a little leverage to get your dowels aligned on glue up but you'd spring the sag out using the adjacent board(s). Gluing to the substrate would be a recipe for disaster if I understand your plan.
    Sometimes I just want to look at pretty pictures,... Thats when I go to the Turners Forum

  3. #3
    Yes, that might work. Or biscuits. I was just thinking that there wouldn't be much wood movement if the wood was encapsulated by epoxy resin anyway. I know we are all conditioned to allow for wood movement, but maybe there are some situations where we can get away with things.

  4. #4
    Whatever you do with the stuff you need to get any twist out. If the stock is reasonably out of wind (not twisted) to start with you could use a hand plane to knock down the worst spots, if it is more gnarly you will have to rip it at which point you can use your jointer. When I am trying to get maximum thickness out of bowed wood I flatten the convex side initially (ignoring cupping). For whatever reason that works best for me. You may be able to get by with just thickness planing depending on the shape of the wood. You can accept some bow and pull the glued-up counter down to whatever it sits on with slotted screws.

    A solid wood countertop needs to be fastened to allow movement, no matter what you slather it in. If you do an epoxy coating, do it on both sides. If you want to glue to a stable substrate you need to resaw your boards into veneers < 1/8" thick.

  5. #5
    Join Date
    Mar 2003
    SE PA - Central Bucks County
    What you absolutely DON'T want to do is to glue that walnut to an MDF substrate or any other substrate that isn't walnut or solid wood with similar wood movement characteristics. You can use metal, composite or wood reinforcement to help force the countertop flat, but they must be attached with mechanical fasteners that allow for seasonal movement across the grain by the walnut itself. If you need to keep things to the 4/4 thickness (I'm assuming you mean a real inch rather than the typical 3/4" finished thickness of 4/4 material, you can easily route a channel for something like thin metal C-channel which will provide enough rigidity to do the job. If the wood is only 3/4" thick actual, fastening the metal securely with mechanical fasteners will be a challenge because there will not be much material there for the fasteners to grip into.

    The most expensive tool is the one you buy "cheaply" and often...

  6. #6
    Join Date
    Dec 2008
    Biscuits may help a little with alignment on glueup but they will do nothing or countering the "sag" from board to board. There is too much slop and too little strength in the biscuit to do anything with regards to getting them flat or keeping the boards registered flush to a face in an attempt to minimize your final flattening (sanding planing, watever)

    Second, this foolish notion the epoxy rivertable world has foisted on their customers that "encapuslating" anything in epoxy suddenly makes it "not wood" or "never going to move" is a complete and utter lie they have put upon their customers and themselves in an attempt to justify using less than dry material and the issues of ignoring wood movement. Boat builders have been "encapsulating" wood in resins for years and the moisture still gets it. Its simply a lie. It may take longer, but its not an excuse to ignore the fundamentals of wood construction.

    Dont be one of those idiots.
    Last edited by Mark Bolton; 05-03-2020 at 5:50 PM.
    Sometimes I just want to look at pretty pictures,... Thats when I go to the Turners Forum

  7. #7
    Join Date
    Jun 2008
    So Cal
    I agree with Jim and others. Mother Nature will keep you honest if you try that plan. Thereís no cheating it would be interesting to see what happens. I predict you wonít be happy.

  8. #8
    You could make a strip laminated "butcher block" style countertop 1 1/2" or so thick by edge jointing and ripping your stock to the new thickness and alternating the bow in each strip. Include a something extra that is dead straight in the glue-up.

  9. #9
    We regularly glue up bowed boards to make table and countertops. Just alternate the bows and flush up the tops. It shouldn't be hard to coerce 4/4 into compliance. You may have glue up in stages to allow time for flushing up the boards. Screws will easily hold down a less than flat top. Also, I like to rip bowed boards in half, flip one half, then glide it back together. This cancels out any bow.
    Last edited by johnny means; 05-03-2020 at 8:23 PM.

  10. #10
    Join Date
    Nov 2009
    Peoria, IL
    Is a 1/4" thick layer of epoxy going to move with a couple 8-10" boards? I thought epoxy was rigid. 2' of flat sawn wood is going to move 1/4" from season to season. MDF won't move a tiny fraction of that. I don't see any good coming from your plan.
    Last edited by Richard Coers; 05-03-2020 at 10:24 PM.

  11. #11
    Join Date
    May 2009
    black river falls wisconsin
    if ya do the mdf route please come back in year or two to post the pics before ya get to redo.

  12. #12
    Have you used an epoxy resin with bad results? And no, I'm not planning to float it down a river

  13. #13
    While I would stay within the boundaries of conventional wisdom when it comes to wood movement, I have seen all of these rules broken and still worked out just fine.

    If you can fasten a plywood substrate to the top of the cabinets and fasten the glued up walnut slab from the underside youíll probably be okay.

    Years ago I bought a house from a builder who ran out of money to put some of the finishes on. As part of the deal he was to install tile on the kitchen counters (never again BTW). On an island we wanted a maple countertop. He brought back several 6Ē wide 4/4 pieces of maple. He glued them down to plywood with liquid nails, because thatís the kind of guy he was. Then nailed in from the top of the maple. The edge was bullnose tile with a grout joint between the tile and the wood. Yes, he actually did this.

    I lived in that house for 14 years. After a few years I cut the failing grout out and replaced it with a more elastic version. The tile stayed adhered, in spite of being boxed in on all sides with the bullnose. The elastic grout stayed in tact for the duration of my ownership.

    I certainly donít suggest trying this at home, nor does it represent good craftsmanship. A 1/4Ē of sag, however, in a 3/4Ē board over 7í isnít likely to matter when you can mechanically fasten that board to a substrate. Itís likely you can remove most of that bow when you edge glue the boards together.

  14. #14

    "Have you used an epoxy resin with bad results?"

    Well, I haven't had any failed edge joints since I realized that the surfaces should be sawn or sanded, and I haven't had a mix kick off in the pot in a long time, but I am pretty conservative about wood movement. I have seen problem projects such as a maple countertop with randomly oriented 1/4" thick maple tiles delaminating in spots. Epoxy is a reliable adhesive and coating if used correctly but it's not a miracle product that overcomes poor design. If you glue 1/2" thick walnut to mdf and encapsulate it with epoxy you may get away with it but you are taking a gamble on problems down the line.

  15. #15
    One thing I do wonder about in this case is this. Does it make a difference that the wood apparently took a set from how it was stored vs it just "wanting" to take a set due to internal stresses? I'd think that it would and maybe he'd have at least somewhat more forgiving wood to work with in this case. Or to put it another way wood that wanted to warp would be worse than wood that would otherwise be straight but was stored poorly and took a 1/4" sag. I'd think the latter would be much more forgiving. Not a license to break the rules willy nilly, but I'd think he has a little more slack.

    Still I don't like the mdf and epoxy idea regardless of the wood's issues or lack of them.

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