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I wish to patch/fill a knot hole in a pine panel at a jointed edge. The hole is only a few mm deep. I don't think doing a sawdust + epoxy/shellac/glue filler will sit well with an oil finish, and I've managed to do without it this far -- so I'm considering trying a special patch.

What is the best way to make a dutch patch containing a knot (or even better, a semi-circular knot!), without the branch loosening out of the patch as I cut it?

elongated hole photo

It's knotty pine, so having just that one filled might look odd, hence why I'd prefer a "knotted" patch over a sawdust fill:

enter image description here

*Edit -- Fixed it. Reporting on results *

Following advice in the answers and comments, I've accepted to use non-wood products into the table. I figured that in the worst case, I could still do the patch. But I am quite satisfied with the results.

Materials:

  • A small tube of "Dark Umber" artist oil paint bought at local craft store.

  • A "syringe" of clear 5-minute epoxy bought at the local hardware store

Steps:

  1. In clean plastic container add, in one corner, a dab of (yet unmixed) epoxy, and in another, a smaller dab of paint. Using a toothpick, stir gradually more and more of the pigment until desired color is obtained. Create a gradient (mess) of colors to pick from. (full res)

    mixture

  2. Mix in fine sawdust from a scraping session, which yields a more matte and opaque result. Without sawdust, the cured epoxy is very vitrous and transparent. You can also obtain opaque epoxy without sawdust by using more pigment.

  3. Use the toothpick as an applicator on the work piece. Disturb as little as possible what is already filled when adding more. I accidentally created mini air bubbles when trying to "pack" more in. (full res)

    application

  4. Waited 16h (the recommended full-cure time on the product), and removed most of the cured excess with a sharp chisel. Then card-scraped the rest. I'm pleased with the results. (full res)

    filled and scraped

    Note: The addition of oil-based pigment to an epoxy resin reduces the final hardness of the cured epoxy. This might be important to consider when using epoxy as an adhesive bond between two objects -- but it doesn't matter as much when used as a filler. In fact, the cured epoxy obtained is much harder than the surrounding wood -- can't sink or scratch it with fingernails.


Similar questions:

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I don't think doing a sawdust + epoxy/shellac/glue filler will sit well with an oil finish

Actually they're fine. Fills don't have to absorb the oil in the same way as the surrounding wood, especially very small fills.

I hate to recommend it but this is a good candidate for commercial wood filler. If you have one on hand that's a colour you could live I don't think you should hesitate to use it.

I don't keep any commercial wood fillers so I'd just fill this with an epoxy mix. I keep a range of wood dusts for this purpose so I can make up a range of colours as needed, but if you only have the nearly white dust from sanding pine you can use that and tint it brown with a little bit of a suitable oil-based paint*.

I'd be aiming for a colour that reasonably approximated that of the nearby full knot. I'd expect this repair to be easily seen, and filled this way viewers will just accept it as another knot without question. If you nail the colour they could look closely and still not be able to tell it's a fill!


*If you would have to buy some oil paint specially for this I guarantee it has further uses in woodworking so it's a good investment. And the paint will last nearly indefinitely — 20 years easy! — if the cap is a good fit and the tube remains undamaged.

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    Since the bottom of the hole is covered in bark/cambium I'm thinking clear epoxy would just take on that color. I'd be fine with seeing through the hole, as long as the cavity is filled. Regarding oil paint tint: I didn't know oil paint could be mixed with epoxy. -- you mean canvas paint tubes, like Indian Yellow or Phtalo Green? – ww_init_js Dec 29 '17 at 5:24
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    @init_js Yes oil paints as come in tubes, any oil-based enamels or trim paints too. They mix readily with epoxies but you don't mix in too much or it undermines the strength of the fill (not that it would matter greatly on a tiny fill, but for structural fills it's important to keep the mix strong). Re. using clear epoxy, that would work here you're right, but I've found it very difficult to keep the mixture free of bubbles during mixing and also it will yellow after some years of light exposure. – Graphus supports Monica Dec 29 '17 at 7:51
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    Thanks for all your help. I tried the epoxy mix. Added result photos. I tried filling the bubble with another layer of epoxy, but it won't stick to the cured epoxy. Not perfect, but good enough. – ww_init_js Feb 2 '18 at 5:44
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    @init_js Good tip for the next time you want to fill with epoxy or an epoxy mix is to preheat the area with a hairdryer (or heat gun, carefully). The epoxy will liquefy when it touches the warm wood and the lower viscosity ensures good penetration and helps it release any air bubbles. This is particularly valuable for finer cracks which are hard to fill deeply with epoxy normally, but heat the wood first and it just sinks in like magic. – Graphus supports Monica Feb 2 '18 at 6:10

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