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Thread: Gap/crack filling methods and alternatives

  1. #1
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    Default Gap/crack filling methods and alternatives

    I am nearing completion of stripping my mahogany canoe hull, and have begun preparing to start fairing. As a result of milling my own strips from rough cut lumber (and my inexperience) I have some moderate filling of gapes and cracks to do. My question is: in-lieu of mixing wood flour or filler with epoxy and patching the gaps that way, could I use a burn-in stock to do the same? I have used Mohawk burn in sticks before, and feel that I could get a fairly close color match, close enough for me anyhow, so I was just wondering if there is any reason this wouldn't work, such as the possibility that the epoxy may not bind to it during glassing. How about mixing wood flour with titebond III?
    Does anyone have any knowledge or experience with this?
    Thank you for the help as always!

  2. #2
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    Default Re: Gap/crack filling methods and alternatives

    It's not worth the risk of incompatibility , stay with epoxy. Tape back of cracks to avoid spills.
    Last edited by andrewpatrol; 03-05-2017 at 11:04 PM.

  3. #3
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    Default Re: Gap/crack filling methods and alternatives

    It's not worth the risk of incompatibility
    Agreed. Also keep in mind that wood flour mixes can be pretty hard, much more so than the strips themselves. This can make it quite difficult to sand the fills fair without dishing the surrounding wood in the process. You may want to add some microballoon filler (WEST 407, etc.) to your mixture to soften it somewhat and make it easier to sand. Color-wise, the wood flour will end up much darker than the wood it came from. It's essentially all end grain. Mahogany dust will come out nearly black when saturated with epoxy or glue. Do some testing first for color. You may need to add something lighter colored, like maple, birch or poplar sanding dust to adjust the final color of your filler.

    I'd also suggest staying with epoxy or Weldwood. A blob of Tightbond filler (or similar carpenter's glue) might take a couple of weeks or more to actually harden completely. About the only wood glue that I would use is Weldwood Plastic Resin Glue (tan powder, mix it with water until thick like pancake batter, dries dark tan color). It is an excellent glue for the stripping part of the building process. It has really good gap-filling capabilities, it's pretty inexpensive, reasonably strong and it sands very well.

  4. #4
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    Default Re: Gap/crack filling methods and alternatives

    If you are talking about Mohawk shellac burn in sticks, no. They are too brittle and won't adhere. I also second what Todd said about wood flour/glue mixtures. I made this mistake on my first woodstrip canoe and despite using a sanding block I could not level the filled area. It was so hard it resisted the sand paper while the surrounding area wore down, making the problem worse. West's filler is designed to be sandable.

  5. #5
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    Default Re: Gap/crack filling methods and alternatives

    Ok, thanks for the insight; I'll pinch my pennies elsewhere!

  6. #6
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    Default Re: Gap/crack filling methods and alternatives

    All kinds of things have been used to glue strips when making a stripper, a number of which such as hot melt glue don't bond well to epoxy. The small area covered by the burn in sticks will not have any discernible effect on the overall strength and stiffness of the boat. The sticks are compatible with varnish, and will be compatible with epoxy. Most of the strength is in the wood in the long direction and glass laterally.

    If you are worried, make some 6x6" wood strip and fiberglass panels. some with and some without a 2"(?) long gap in the center filled with a burn in stick, support it on its edges and see how much weight it takes to break them. I doubt that you will see much difference.
    Management is the art of counting beans. Leadership is the art of making every being count. --Joe Finch

  7. #7
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    Default Re: Gap/crack filling methods and alternatives

    Dave, I think I may have 1 or 2 gaps in the whole boat that span 2", most "gaps" are just chatter marks from the router or similar, and only 1 of them goes all the way through and it's not wide. My main concern was compatibility with epoxy aesthetically, not so much structure. If you think/know it is, I think I'll give it a go.
    It would certainly make fairing easier than epoxy and wood flour
    Last edited by Garagebuilder; 03-07-2017 at 07:03 PM.

  8. #8
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    Default Re: Gap/crack filling methods and alternatives

    My main concern for the sticks would not be brute strength, but more the possibility of delaminated fiberglass spots upon being flexed, or from minor impacts. They might be perfectly fine, but I'm not fond of incorporating elements into my boats which "might be fine" when there are other options which have already been proven to work.

  9. #9
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    Default Re: Gap/crack filling methods and alternatives

    That was my main concern as well, that's why. Did check on here to see if anyone had tried. They make epoxy based sticks which is would think would bond. Do you think wood flour filler will age better with time if made from primarily the same wood? I know mahogany darkens as it ages

  10. #10
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    Default Re: Gap/crack filling methods and alternatives

    Quote Originally Posted by Todd Bradshaw View Post
    My main concern for the sticks would not be brute strength, but more the possibility of delaminated fiberglass spots upon being flexed, or from minor impacts. They might be perfectly fine, but I'm not fond of incorporating elements into my boats which "might be fine" when there are other options which have already been proven to work.
    That is a valid concern if there is a large spot coated with a material that doesn't bond well to the epoxy. I think that the intent is to fill the small gaps and clean off any of the burn in stick coating the surface surrounding the gaps. I think that in this case the gaps are very narrow and any excess would be sanded away.

    Todd is what we call an expert, on account of he actually is one, having built a lot of well made strippers, and having actually paid attention to what was going on the whole time. Where I disagree here is about the assumptions concerning the extent of the affected surface area. If it was too large to ignore, I would have to agree.

    A trick that I learned when I was about 6 was hiding a small gap in a glue joint by filling it with Elmers wood glue (pre-Titebond) and sanding while it was still wet. The dust mixed with the glue in the gap and the color match was good with pine, at least to my 6 year old inexperienced eye. It worked with small gaps and a minimum amount of glue. You have to remove all of the glue outside of the crack. I find that wood flour darkens considerably when mixed with epoxy.

    1 or 2 gaps in the whole boat that span 2", most "gaps" are just chatter marks from the router or similar, and only 1 of them goes all the way through and it's not wide.
    Is that all? No worries.
    Management is the art of counting beans. Leadership is the art of making every being count. --Joe Finch

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