Filling End-Grain Cracks in Slabs

Finishers discuss reinforced epoxy and similar crack-filling materials. September 29, 2009

I'm building more slab tables these days, and I'm wondering what to do about end grain cracks. These are large (as wide as 1/2"+ and 12" long). I don't want to butterfly these, as is the fashion. Will an epoxy work to fill and secure these cracks? I want anything clear (preferred) or pigmented to fill and secure. I'm leaning towards West Systems Epoxy with the 207 hardener, but I'd like to know if anyone else has tried something that works for them. Thanks in advance!

Forum Responses
(Finishing Forum)
From contributor A:
Epoxy has the only chance to span 1/2". You may want to play around with some milled fiberglass powder mixed in with the resin. That is a pretty wide gap and epoxy resin is brittle by itself. The milled (not chopped) fiberglass looks like a fine white powder. However, it is still fibers that interlock to reinforce the resin.

At some point you have picked up a piece of plastic (usually glass reinforced nylon) without realizing it. They make things like bicycle tire irons and some tools (Paslode pneumatic nail guns) in this fashion. They are very strong even though the fibers are extremely short. If the resin turns white you could add some West System pure graphite powder to turn it jet black. Definitely experiment with a slotted piece of plywood before hitting your slabs.

From contributor G:
You can use the WS Epoxy, it will take a few tries to get it to fill level. The first thing you need to do is to use a brush and coat the inside of the crack to seal off the grain. Let it dry. Sand the surface to get the wax off, and then build a dam out of tape to contain the epoxy. Then fill it up. If it levels out for you great, if not, use a third level and that should do it for you.

From contributor O:
Second choice is tinted automotive Bondo. It is the standard stuff auto folks use under their finishing and it has a bit of elasticity to it. If the wood is not pretty much done drying either will fail. During application Bondo stays put a little better (it is hard to get enough filler into epoxy to prevent sagging). Some clay or duct tape can be used to make a dam to corral the stuff until it sets. In my experience Bondo does not clog sandpaper as fast as epoxy, but if you get to the epoxy at the right place in its cure a scraper works real easily.

From contributor A:
Bondo is made from polyester resin. It is more brittle than epoxy. The fairing compound's sandablity is based more upon the filler than the resin. If you mix micro ballons (phenolic hollow spheres) or glass microspheres into epoxy resin it will sand as well as or better than Bondo or even Microlight (NAPA's version with microspheres). The polyester sands easier faster. If your wait a couple of days they sand the same. Bondo (polyester fillers) are more suited for coating and filling holes or dings. Epoxy bonds much better to wood than polyester. I believe epoxy would be better at spanning these large gaps.

From contributor O:
No problems, I did give it second place. The 18 inch by 65 inch by 4 inch thick air dried Indiana black walnut sofa/magazine/cocktail table, with a Jorgensen vise on the side workbench - that I eat dinner in front of the TV on and cleverly converts to a seating bench by rotating the base. I made it back in college, and it sees use daily. It has Bondo filling in a few holes on the top, in several end cracks and filling the 1 inch holes in the side that that resulted from holding the two boards together with ready bolt. I usually use epoxy now for heavy tops, but the 35 year old filler in that table is as good as the day it went in.

Bondo from the auto store and a tube of tint is likely cheaper than two cans of West's epoxy, pumps, micro balloons, color and shipping. I buy either West or Aero by the gallon and it lasts so long that I don't remember if it ships with a hazmat fee or not. I havenít had any problems.

From contributor J:
We use a slow set epoxy for filling knots and cracks. The first thing that I do is to knock any and all of the loose material out of the knot or crack with a scratch awl and air blower (very important). The next thing I do is rub or brush a little sanding sealer around the outside of the knot or crack. Then I tape off the back with blue tape and anything that could let it out the side. I also put tape on top around the defect. Now mix up your epoxy, the slower set the better off you are. I usually try to do this just before quitting time so no dust lands in it while it is wet. After you mix your epoxy put a little bit of dark brown or black paint pigment in it.

Now put the stuff in a small paper cup and have a few small 1/8" sticks handy. Heat the board up with a heatgun and then pour the mix into the crack. With cracks 1/2" wide I would not try to fill them up flat but let the epoxy just start to creep up to the top surface but still dish down in the middle. As it flows you might get some air bubbles. The heatgun and small sharp sticks help but you have to be careful not to blow the stuff all over the door. If you want to fill them up flat you can but you have to watch for bubbles. As the stuff sits it will seep into the cracks and the level will fall. I know this sounds like a big pain but it makes really neat looking knots and lets you use lumber you normally couldn't use. I have filled knots in black walnut doors that were 2" x about 5" and the customer thought that was the coolest thing ever!

From the original questioner:
Thanks for everyone's help! I ended up using epoxy from Douglas and Sturges. I used 7132 resin and 2001 hardener. I'm keeping it clear. I tape off the holes with brown clamp tape (brown paper packing tape). I drag a propane torch across the bubbles to pop them. Looks great so far!

From contributor J:
You might consider coloring you epoxy so that light does not shine through the crack. How long do you have to work with your epoxy before it is hard?

From the original questioner:
This epoxy is slow. I was still able to pop bubbles for hours. I avoided color because I wanted the slab to look the same without the inconvenient and unstable cracks and holes.