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  1. #1
    Join Date
    Jul 2017
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    Launceston, Tasmania
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    Default Filling knots with clear epoxy :( help

    Hi all,

    Wednesday night I did a small job filling some imperfections with a barnes easy cast clear polyurethane on a blackheart sassafras slab, below are the results.

    As you can see they're far from acceptable, I've used the product once before and had far better results, that was with tas oak. I had been using a heat gun and when I left it the resin looked great. It was 95% clear of bubbles and most were only on the surface where I could sand them out. I can only assume that overnight air bubbles were forced out from between the fibres of the timber as most of the bubbles are at the bottom of the cast, is this possible? or is it more likely that the temperature of 7 degrees overnight affected it?

    Either way what's the best way to remove it, I'm really only concerned about pictures 3, 4 and 5.


    IMG_20180607_190603.jpgIMG_20180607_190619.jpgIMG_20180607_190632.jpgIMG_20180607_190638.jpgIMG_20180607_190700.jpgIMG_20180607_190710.jpg

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  3. #2
    Join Date
    Jul 2005
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    Oberon, NSW
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    Default

    There can be a number of reasons for those bubbles to form. Moisture in the timber, temperature, even changes in barometric pressure, just to name a few.

    G'luck trying to fix it... for me the best method to remove it is to simply pick it out. Bit by laborious bit. <groan>

    I've found that on larger pieces where I can't use a pressure vessel (whether vac pot or otherwise) it's best to fill knots by doing a very small pour first, brushing it over the sides of the knot to seal it and letting that cure at least partially before filling the rest.

    (How long I let it cure depends on the product. With some epoxies I've had delamination problems if I let the seal coat cure properly. YMMV. I haven't used the Barnes stuff, so can't comment.)

    Even then, sometimes bubbles appear in this "seal coat" but at least they're easily removed with a pick and are generally invisible when filled the rest of the way.

    I guess this is one of the reasons I'm moving more'n'more away from clear fills to mixes such as coffee-grinds and epoxy. Much harder to see the bubbles...
    I may be weird, but I'm saving up to become eccentric.

    - Andy Mc (AKA "Ghost who posts." )

  4. #3
    Join Date
    Jul 2017
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    Launceston, Tasmania
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    Default

    Thanks for that, I'll pick it out then and start again. Worst part is I'll probably have to buy another pot of the stuff.

  5. #4
    Join Date
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    Tallahassee FL USA
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    Default

    A similar problem came up on the AAW forum recently. The culprit was considered to be moisture in the timber, brought to boiling by the heat of curing the resin. The preferred solution was to bake or nuke the timber to zero moisture beforehand.

    To fix it, I reckon you'll need to excavate the old stuff, e.g. with a Dremel.

    I also favor mixing coffee grounds in the epoxy. Over-fill the final coat and sand flush. The coffee grounds typically contain light and dark elements; when sanded flush, it appears as miniature Terrazzo and is quite handsome.

    Cheers,
    Joe
    Of course truth is stranger than fiction.
    Fiction has to make sense. - Mark Twain

  6. #5
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    Default

    I'm thinking if I just dig out the top 5mm and then fill it back in with coffee grounds that should work well. I wasn't set on a clear cast, it's just what I had and I hadn't considered colouring it. Do I need to do anything special to avoid delamination?

  7. #6
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    That sounds like a plan, although personally I'd pick out as much as I have the patience for; all it really needs is a clean surface to key to. I simply use an artists' brush and a straw to blow out any debris.

    FWIW, I've found that the cheaper 2-part epoxies - such as Araldite - work well with coffee grinds. The exxy stuff I save for what needs to be clear cast.

    I wouldn't use Araldite in this case though. Not layering one type of epoxy over another.
    I may be weird, but I'm saving up to become eccentric.

    - Andy Mc (AKA "Ghost who posts." )

  8. #7
    Join Date
    May 2011
    Location
    Melbourne
    Posts
    188

    Default

    If you ever need to buy some more, try West System Epoxy. There are other brands but I've found WS to be fantastic for this sort of thing.

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