Filling knots with epoxy

[Brian Hale]

Hey Folks!

I'm working on the floor for the new shop, laying "Utility"grade unfinished oak flooring and some of the boards have defects like knot holes and such, some about 1" in diameter. I like the look i want to fill them.

Is epoxy the way to go?

My only real experience is with those 2 part syringes you get from the borg and small repairs, nothing this big.

I see System Three offers various fillers like Wood Flour, do i need them for theses holes?

Thanks!!
Brian :)

[David Cefai]

I would recommend that you use a filler with the epoxy.

1. Filler is cheaper than epoxy
2. It will thicken the mix, making it easier to apply.

However experiment to make sure that you can get the colour you want.

From bitter experience let me point out:

1. When they talk about the mixture heating up and hardening too quickly - they really mean it!

2. Be scrupulous in proportioning the components. If you get it wrong the stuff will not harden. In that case Acetone does a great job of removing the resultant gunk.(Normal solvent precautions apply)

If you need to level off the epoxy, do it when it has just set sufficiently. At this point card scrapers or handplanes work very well. Later on it's another story.

[John Keeton]

Dry coffee grounds make an excellent filler with the epoxy, and will give you a very dark area that will have the same appearance as the knots. The System Three is good stuff, and can be worked in about 12 - 14 hours pretty easily. As David said, after that, it is going to be much more difficult.

[Bas Pluim]

Definitely use a filler, either something commercial or make your own. It will improve the strength because there is less shrinkage compared to pure epoxy.

As John suggested, coffee grounds work well for making dark knotholes. Fine sawdust is good too. I've made blends of coffee grounds, saw dust and cocoa powder to fine-tune the look (and now, a dash of paprika....).

Get a cheap handplane and a large "beater" chisel to do the leveling, because some gunk will get on your tools if the epoxy isn't fully cured yet (typically 24h). I use a solvent and an abrasive pad to clean it off.

Harbor Freight sells a small digital scale that's perfect for measuring the ingredients.

For very large knotholes, consider putting in a plug by drilling out a section with a Fortsner bit and making a plug with a hole saw.

[Steve Griffin]

Ink, dye or black or brown enamel work great for color too. I use the little Testors hobby paints for the shop, and it might even be plenty for your floor. Just takes a tiny amount.

-Steve

[Aaron Wingert]

I agree that epoxy with various fillers works well. I do coffee grounds on a fairly regular basis.

But, why not use a router and install nifty looking patches using a template. Perhaps bowtie inlays would be a really cool accent that wouldn't stand out to most folks if you use wood with similar grain and coloration for your patch. Personally I think in a shop I'd use the same wood, but would orient the grain offset enough that the patch shows just enough for other observant woodworkers to notice it!

[Brian Effinger]

The Wood Whisperer recently made a short video on fixing knots with epoxy and dye. Check it out here: http://thewoodwhisperer.com/fixing-a-knot/

[Henry Ambrose]

Use multiple passes to fill the holes. If you pour the whole thing at once you are likely to get bubbles when the mass of the plug heats up as it cures. Sanding dust form the wood you are filling will give a nice tone match - darker - but the right tone.

[Richard M. Wolfe]

Believe it or knot:D, I've had pretty decent success making knots. I find a knot that is close in size to the hole or even make one using wood of the same species. Since the knot is end grain using "non-knot" wood usually matches pretty closely when a finish is applied. If the knot fits pretty tightly (I've had to tap them in before) wood glue works to hold them in place, or if it's a loose fit epoxy may work better.

[Brian Hale]

Good Stuff Guys!

Thank You!

Coffee grounds? Must admit, i'd never thought of that but it makes sense. Stains everything else......

I've got about 40 places in need of filler but only a couple are the thumb sized ones so it shouldn't be too bad. Making the filler black is just what i was looking for so i'll go that route.

How about the sanding dust from an ROS? Will that work good or is something more corse in order?

Thanks
Brian :)

[Tom Matthews]

How about the sanding dust from an ROS? Will that work good or is something more corse in order?That's what I always use. Chances are if I'm filling knots I've been sanding the same wood so I've got the correct color sawdust. I keep extra sawdust in babyfood jars for those times when the sander isn't out.

[Rod Sheridan]

Hi Brian, you've received some great advice on how to solve your problem, however there are certain dangers with using epoxy.

Skin contact with un cured epoxy is hazardous, you will need some form of plastic glove.

The acute effects of contact are often cumulative, with lower and lower doses required to produce the same effects.

Heating and sanding produce these hazards as well, as the enclosed quote from West Systems Epoxy indicate.

"Partially Cured Epoxy Dust

Sanding partially cured epoxy produces airborne dust, which increases your risk of exposure by skin contact, inhaling or ingesting. Although epoxy is firm enough to sand within two hours, it may not cure completely for up to two weeks. Until then, the dust can contain unreacted hazardous components. Do not overlook or underestimate this hazard."


I'm not suggesting that you not do this, I use epoxy for knot filling in turnings, however sanding has to be done with a method of containg the dust, and making sure that the epoxy is cured, and not heated by sanding. (Sand by hand, or in short time intervals with a cooling period if using powered sanders).

Regards, Rod.

[John Pennisi]

Rod,

This stuff is scary. I wear a mask but always sand/scrape epoxy 1-2 hrs after it sets. Are you saying the dust is less safe until the two week mark?

John

[Mike OMelia]

I use Z-poxy. Shrinking, never seen it. Epoxies are not solvent based.

[Jim Andrew]

Wondered if I was the only one turning knots to fit into holes? In panels, I take a round file to remove the bark inside the hole, and smooth it a bit, then turn a piece to fit. I glue it in, tap it with a hammer to get it tight, and when it is dry, saw it off a little long, then sand flush.

[Jim Foster]

I never filled a knot hole that did not "shrink." If the epoxy is not the culprit (most epoxies only shrink about 3.5%), it must be that it continues flowing into the cracks and crevices after it is first leveled, or it could be air bubbles coming out and letting the epoxy settle. Either of these conditions gives the appearance of shrinking.


I use Z-poxy. Shrinking, never seen it. Epoxies are not solvent based.

[Jim Finn]

I use Z-poxy. Shrinking, never seen it. Epoxies are not solvent based. I also use Z-Poxy. Get it at my local hobby store.