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Timber Benchtop cupping

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  1. #1
    Apprentice (new member)
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    Default Timber Benchtop cupping

    Can some one help a novice out.

    I laminted some 19mm x 200 thick furniture grade Jarrah together for a bench top 1500x 900 x 19 which i finshed one side in Wattle 708 epoxy , I then liquid nailed and srewed some 18mm ply to the back to give some thickness to the bench top and Ran a jarrah edge around it.. Since I only sealed 1 side not knowing that this would increasse cupping , also dur to the over hang of 400 on one side and one end to allow for seating - I am now left with my beautiful bench top cupping and not shore where it will stop.. do a put a flat bar to try to resist further movement or do I take it offf and seal the other side , how do I remove the cup if possible before sealing/ steam/ water/ weights .. can some one give me some pointers as it took a long time to make the top in the first place and looks outstanding..
    Steinos

  2. #2
    Resigned SilentButDeadly's Avatar
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    Not much you can do to fix this.....having the ply glued on the back makes it hard.

    Essentially it is unsealed, still a bit too thin and on the move. Sealing the plywood backing might slow its responses to atmospheric humidity down a notch

    Flat bar won't fix it....it'll just bend too. You'd need to try thick steel angle (50mm at least) instead. This might prevent it moving too much further into the future...
    Joined RF in 2006...Resigned in 2020.

  3. #3
    1K Club Member Master Splinter's Avatar
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    Take the top off, strip off the plywood, scrape off the liquid nails (and never use it for furniture grade woodwork again) and try again, using (prefereably) an epoxy adhesive or decent crosslinking PVA.

    MDF would make a better substrate than ply, but I'd prefer to do the whole thing in jarrah. Alternate the orientation of the growth rings (up/down/up/down/up/down) so that cupping will tend to cancel itself out, and make sure you seal both sides when finishing.
    DIY electrical wiring to AS/NZS3000 - details here - http://goo.gl/9d33T (PDF file)

  4. #4
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    Default Moving forward

    If I was to strip off the ply and liquid crap and back it in jarrah and seal with epoxy.. with the current cupping .. is it possible to remove that cupping or reduce it prior to backing it , as would the backing work if glued it to a cupped surface ??..

    I have another long section that is only 600 deep by 3 meters with my cook top and sink in it but there is no movement evident.. Hopefully it stays that way

  5. #5
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    I worked once in a place that had a thicknesser that could take a whole table top through at once. I you can find somebody with one of those that will help you out that would be one way of getting it flat. It will of course be thinner though.

    me
    Life's too short for dull sandpaper

  6. #6
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    The comments before are all hitting the nail on the head alternating the growth rings is so important. There is also a reason why Boral Timbers and like produce their T & G hardwood flooring & decking 19mm thick x 85mm wide, any wider the timber tends to cup. Your 200 wide x 19 is a bit adventurous

  7. #7
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    Yep ya learn by your mistakes.

    You say "using some flat bar", yes this can help I would use 40mm or 50mm wide x 10mm thick flat bar or some 40x40x6mm or 50x50x6mm angle iron.

  8. #8
    1K Club Member Master Splinter's Avatar
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    Mist the inside of the "C" with water, then plunk some weights on it to provide a gentle straightening force and leave it. Check and rewet daily.

    My rule of thumb for wide cupped things is that if you can take the cup out with light to moderate hand pressure, you can use wood/iron bar/concrete safety rail/whatever to restrain them in the future. If it's a bit of a struggle to get flat, then there's a good chance it'll crack when forced flat and restrained.
    DIY electrical wiring to AS/NZS3000 - details here - http://goo.gl/9d33T (PDF file)

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