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Thread: Dealing with defects

  1. #1
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    Dealing with defects

    I have a piece of black walnut, about 58" x 23.5" x 2", that I'd like to use for the top of a stand-up desk, and I'm trying to figure out how to deal with the defects. The finish will be shellac; French polished if I can manage the patience to do a surface this large.

    Here's a view of the side that, I think, will be the top:
    2015-07-22 17.09.13.jpg

    Here's a closeup of the knots shown in the top half of the picture above. Can I can fill these with epoxy, maybe tinted?

    2015-07-22 17.09.22.jpg

    The problems on the right-hand side in the first picture trouble me a bit more. Here's the crack about 2/3 down on the right side. If I have the terminology correct, this is a shake rather than a check. . . the crack seems to follow the grain boundaries. (That's more apparent when looking at it from the edge; I didn't get a picture from that perspective.)

    2015-07-22 17.10.14.jpg

    The upper right corner bothers me a bit, too. There are the cracks, and the sapwood in that area seems to be particularly soft. I'm inclined to rip about two inches off the right hand side of the board (the desk will sit against a wall, so a straight cut on that edge isn't a bad idea anyway) to get rid of the cracks in the picture just above, as well as this corner. That will leave me with about 21" of width -- a bit narrower than I'd like, but still manageable for its intended use. Thoughts?

    2015-07-22 17.09.54.jpg

    Then there are some problems that show up on the other side, which I think will be the underneath of the desk top.

    2015-07-22 17.10.51.jpg

    Specifically, there are cracks (checks) at both ends. I'm thinking of sealing these with epoxy and adding butterfly keys to keep the cracks from propogating. Will that work?

    2015-07-22 17.11.02.jpg and 2015-07-22 17.11.13.jpg

    Any advice will be greatly appreciated.
    Michael Ray Smith

  2. #2
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    Fill with epoxy, as ypu're thinking. I've has excelleny results adding a bit of aniline dye to expoxy to alter the color.
    If the thunder don't get you, the lightning will.

  3. #3
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    Tony, will the epoxy alone stabilize the cracks and keep them from running, or do I need something like a butterfly key?
    Michael Ray Smith

  4. #4
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    Quote Originally Posted by Michael Ray Smith View Post
    Tony, will the epoxy alone stabilize the cracks and keep them from running, or do I need something like a butterfly key?
    A butterfly key (aka dutchman) is likely to do better than epoxy along.

    I have used sawdust from the wood being worked to mix with epoxy to match the wood being repaired. Of course this only works with clear epoxy.

    Another trick is to put Scotch tape over the epoxy. This will create a smooth surface on the epoxy when it hardens.

    jtk
    "A pessimist sees the difficulty in every opportunity; an optimist sees the opportunity in every difficulty."
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  5. #5
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    I'd just use the Dutchman and work the crack with scrapers and such until I liked the result.
    Bumbling forward into the unknown.

  6. #6
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    Butterfly inlays are useful for edge joining boards, but don't forget that they must be installed oriented cross-grain, with the result that they partially restrain the board from shrinking/expanding with humidity changes. Therefore, if the inlays are installed when the board is at its lowest likely moisture content in the environments where it will spend its time, then the board can only expand, and the inlay will tend to keep the crack closed. But if after the inlay is installed, the board shrinks in width, then the butterfly inlay will tend to make the crack worse. And don't forget the internal stresses that caused the board to crack in the first place. These must be dealt with.

    You have three practical choices in dealing with a cracked board, at least if the goal is a stable end-product, that is.

    The first method is to wait for the board to stabilise, and then cut the crack out of the board. Optionally, you an cut out the crack, and then wait for the board to stabilise. Whichever you choose, remember that the board cracked due to internal stresses and those stresses must be relieved either mechanically (cut out the stressed wood), or over time (allow the board to season) by allowing the internal stresses to resolve themselves by cracking the board until the crack stops on its own. Either way takes time.

    Another solution used in commercial applications where time is not so abundant is to rip the board full length removing the crack and (hopefully) relieving the internal stresses, and then gluing the board back together. Of course, this creates a grain discontinuity which you may find aesthetically objectionable, but it is quicker, relatively stronger, and likely more reliable than epoxy and butterfly inlays.

    The third solution is to glue the crack and close it with clamps, hoping the internal stresses are resolved enough to not cause the glue-joint to eventually fail.

    Stan

  7. #7
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    Quote Originally Posted by Stanley Covington View Post

    The first method is to wait for the board to stabilise, and then cut the crack out of the board. Optionally, you an cut out the crack, and then wait for the board to stabilise.

    * * *

    The third solution is to glue the crack and close it with clamps, hoping the internal stresses are resolved enough to not cause the glue-joint to eventually fail.
    Stan
    Thanks, Stan! I appreciate your thoughts on this. Some questions:

    1. How do you know when you've waited long enough for the crack to stabilize? I just bought this piece, but I can probably find out about when it was milled.
    2. If you cut out the crack after it stabilizes (and by that, I assume you mean to cut off the end of the piece where the crack is), don't you actually increase the chances of it cracking again? As I understand it, a stable crack does not necessarily mean that all internal stress is relieved. . . in fact, that probably never happens. There are still internal stresses -- but they balance each other out enough that the wood won't pull apart any more than it already has. If you disturb the wood, isn't it possible to disturb the equilibrium of the internal stresses so another crack starts? (I'm just thinking through this conceptually; I have no experience in it at all.)
    3. Similarly, doesn't closing the crack with clamps actually reintroduce internal stress? It's just that the glue is strong enough to keep the crack from reopening. . . at least immedaitely. Isn't it possible that closing one crack will open another?
    4. Given that, what's wrong with letting the crack stabilize and then filling it with epoxy? I don't think epoxy either shrinks or expands as it hardens, so it won't introduce any new stresses. I think.
    Michael Ray Smith

  8. #8
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    Quote Originally Posted by Michael Ray Smith View Post
    Thanks, Stan! I appreciate your thoughts on this. Some questions:

    1. How do you know when you've waited long enough for the crack to stabilize? I just bought this piece, but I can probably find out about when it was milled.
    2. If you cut out the crack after it stabilizes (and by that, I assume you mean to cut off the end of the piece where the crack is), don't you actually increase the chances of it cracking again? As I understand it, a stable crack does not necessarily mean that all internal stress is relieved. . . in fact, that probably never happens. There are still internal stresses -- but they balance each other out enough that the wood won't pull apart any more than it already has. If you disturb the wood, isn't it possible to disturb the equilibrium of the internal stresses so another crack starts? (I'm just thinking through this conceptually; I have no experience in it at all.)
    3. Similarly, doesn't closing the crack with clamps actually reintroduce internal stress? It's just that the glue is strong enough to keep the crack from reopening. . . at least immedaitely. Isn't it possible that closing one crack will open another?
    4. Given that, what's wrong with letting the crack stabilize and then filling it with epoxy? I don't think epoxy either shrinks or expands as it hardens, so it won't introduce any new stresses. I think.
    1. The best way is to use a moisture meter, and compare the MC of the wood in question with the same species that has been in your shop for a few months. If the new wood closely approximates the old wood, then you should be alright. The other way (traditional way before moisture meters) is to let the wood acclimate to your shop over some months.

    2. You are right about the internal stresses cutting the board can create. But don't forget that you will need to cut the board eventually anyway, and will be faced with the same internal stresses. The sooner those are isolated and resolved, the better. If it cracks further after cutting off the crack, that would have happened anyway, so it is better to find out about it sooner than later. We have all experienced a board that looks great, and has a stable moisture content curve, but when cut to length, it cracks of warps or twists due to internal stresses that appeared to have been resolved already, but in fact were just in balance.

    3. Closing with clamp does create stresses. It is a matter of degree and if the results are acceptable or not.

    In the ideal world (and hobbyists can actually do this) it is best to let boards "rest" after milling, cutting, etc before incorporating them into the finished product. Not always possible, but this old method works for the patient.

    4. Epoxy may work fine. I could not accept the appearance in a visible board, but to each his own. However, keep in mind that epoxy will restrain wood movement too, since it does not expand or contract with the wood during seasonal moisture content changes, and can make a crack worse. See my earlier comments about butterfly inlays.

    Stan

  9. #9
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    Clear epoxy with spent coffee grounds will work in the knots and contained splits.

    I have a mesquite rocking chair seat that is 11 or 12 years old that has splits filled with clear epoxy. It's a good fix. The spent coffee grounds is a technique I learned here on the creek.
    Last edited by lowell holmes; 07-24-2015 at 3:42 PM.

  10. #10
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    Quote Originally Posted by Stanley Covington View Post
    1. The best way is to use a moisture meter, and compare the MC of the wood in question with the same species that has been in your shop for a few months. If the new wood closely approximates the old wood, then you should be alright. The other way (traditional way before moisture meters) is to let the wood acclimate to your shop over some months.

    2. You are right about the internal stresses cutting the board can create. But don't forget that you will need to cut the board eventually anyway, and will be faced with the same internal stresses. The sooner those are isolated and resolved, the better. If it cracks further after cutting off the crack, that would have happened anyway, so it is better to find out about it sooner than later. We have all experienced a board that looks great, and has a stable moisture content curve, but when cut to length, it cracks of warps or twists due to internal stresses that appeared to have been resolved already, but in fact were just in balance.

    3. Closing with clamp does create stresses. It is a matter of degree and if the results are acceptable or not.

    In the ideal world (and hobbyists can actually do this) it is best to let boards "rest" after milling, cutting, etc before incorporating them into the finished product. Not always possible, but this old method works for the patient.

    4. Epoxy may work fine. I could not accept the appearance in a visible board, but to each his own. However, keep in mind that epoxy will restrain wood movement too, since it does not expand or contract with the wood during seasonal moisture content changes, and can make a crack worse. See my earlier comments about butterfly inlays.

    Stan
    Thanks for the education, Stan!
    Mike
    Michael Ray Smith

  11. #11
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    Pulling up this thread from last week. I've been thinking about this a lot before I make the first cut (I paid too darned much for the wood going into this desk to ruin it!), and think I've figured out how I'm going to deal with all the obvious defects except this knot. Note that the board is 2 inches thick, and I'll keep most of that, losing just enough to flatten and smooth it. The knot goes all the way through the board, with the other side showing just small part of the knot and no crack. Probing with a thin wire shows the crack at its deepest goes about 1 3/4 inches deep. I'm not going to be able to trim the board to eliminate the knot -- it's just not close enough to an edge or the end -- and this will be the top side of the board that makes the top of the desk, so I have to deal with it in a way that is attractive and prevents further cracking. And I don't see any way of dealing with it that does not involve a filler.

    My original plan was to fill this with epoxy, maybe mixing sawdust from the same board into the glue, and that may be what I ultimately do. Stan's caution that epoxy used to fill cracks won't expand and contact with the wood, creating the possibility that the epoxy itself can cause cracks to propogate give me pause. I'm not sure how that plays out in a knot like this -- I think the concern about the crack propagating is less than with a crack that runs with the grain, but I'm not sure. But anyway, I still have several questions bouncing around in my head (lots of empty space in there for bouncing).

    First, is a filler the right idea? Or should I think about cutting out the knot and replacing it with a sort of plug? I really don't like that idea, and I'm not sure it would work anyway, but I thought I'd ask.

    Second, assuming I need some sort of filler, is an epoxy-based filler the right idea? Is there another material? I have lots of sawdust already from cutting other pieces of the desk; is there some other binder I could mix with it to make a better filler?

    Third, should I try to fill the entire void with the filler, or should I just try to fill the top part of the void, enough to finish the surface and make sure the filler doesn't fail?

    Fourth, any other ideas?

    [Edited to add the photo. I forgot!]

    2015-07-31 10.57.29.jpg



    Thanks in advance. I don't really have anyone to talk to about these things, other than the clerks at my local Woodcraft store. I really appreciate the help I get from experienced Neanderthals on this forum.
    Last edited by Michael Ray Smith; 07-31-2015 at 11:28 AM.
    Michael Ray Smith

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