Filling Deep Gaps Under a Clear Finish

Most grain fillers either take a long time to dry or turn milky white when applied too thickly. Here are some ideas for a water-clear crack filler. December 10, 2007

I'm finishing walnut slab coffee tables and sofa tables. I'm using Crystalac clear grain filler and finishing with Waterlox. I'm aiming for a glass smooth finish with good clarity and depth, and I'm happy with how it's going except for one problem. A few of the slabs have small but deep gaps. I don't like most grain fillers because they appear chalky - they don't have the translucence or reflective quality of wood grain. That's why I'm using the Crystalac clear filler, but the Crystalac can't be applied thick (I've tried it and it dries gummy). It's taking days to apply a thin layer every few hours. (This one particular gap is about 1/4" deep, 2" long, and 1/8" wide.) Plus, I'm finding out that the Crystalac gets cloudy when it's this deep. I would love to find something that can fill deep gaps and maintain clarity. I read on a luthier's website that System 3 epoxy can be thickened with silica powder and still be glass clear, but I haven't tried it on these weird little gaps, so I don't know if it would work yet. Does anyone have advice on this problem? By the way, I tried planing past the gaps, but there are always more!

Forum Responses
(Finishing Forum)
From contributor B:
I am not familiar with the epoxy you are referring to, but I do know and have myself filled deeper depressions with clear epoxy, then finished over the top with excellent results. Try a sample fill with the epoxy you have on another piece of wood to see if it reacts as you want.

From contributor R:
Have you tried gluing in a wedge?

From the original questioner:
Contributor B, that definitely sounds like what I'm after. When you say "epoxy," do you mean epoxy finishing resin or epoxy glue? Whatever it is, I'm all over it.

Contributor R, a wedge is probably what I ought to be doing with the deepest gaps. I'm hoping to avoid that most of the time, though, because some of these gaps are located at the most beautiful parts of the slabs, like at the crotch, where the figure looks like flame.

From contributor B:
I was referring to epoxy glue. Contributor R's post reminded me of something I have also done. For deep areas, fill with wood or sawdust, then put in the epoxy to avoid having a boulder of material (epoxy in this case) that will definitely not move at the same rate as the wood. A bit safer in my opinion, as it will not heave if the wood moves dramatically.

From contributor K:
You might want to look into a variable mix epoxy that offers more flexibility. Industrial Formulators of Canada offers one that you can vary the hardener to resin for a more flexible joint that moves more with the wood. I think they were bought out by West Systems?

From contributor B:
Good tip, I did not know about those kinds of epoxy. I am actually more of a hide glue person, but the epoxies have their place. Good to know one that is less rigid is available.

From contributor A:
You cannot thicken epoxy and still maintain its clarity. When you mix silica (think of sand that is the smallest fluffy piece of dust that you can imagine, then think a lot smaller), it turns the epoxy white. Epoxy can be used with some success all by itself in situations like yours. Some of the epoxies are quite thick.

I'm curious how the Waterlox is going to look over the Crystalac. Did you do samples before you started on the actual project?

From contributor U:
I've used epoxy glues to fill gaps like that, but there is often an issue with bubbles staying in the glue, making it look milky. I've used torches (very lightly) on thinner applications of epoxy to draw bubbles to the surface to pop, but it doesn't always work in the thicker applications. I've also used the pour-on epoxy finishes you'll find at many hardware stores to fill these gaps. Just experiment on scrap. Also, assume that the epoxy will yellow with time. On a walnut table with a Waterlox finish, this isn't too much of a concern.

From the original questioner:
Thank you for the tip about variable mix epoxies. I did not know about them. Also thank you for steering me away from thickeners. I would have hated myself if I had filled it with something that dried white.

By the way, the Crystalac looks incredible under the Waterlox when it's used to fill normal pores. But when I apply it deep, it turns cloudy.

And thanks for the bubble warning. Would a hair dryer serve the same purpose? Or maybe the wind from the hair dryer would cause problems. Have you ever tried it?

From contributor C:
Seems to me that the old standby (shellac stick) would do this best and fastest. I use clear sticks mixed with a bit of color to get the deep wet look.

Another option is to create a dark knot or streak (both common in natural walnut) which can be painted over any filler (bondo or epoxy putty would be simplest). When these are done well, they are more attractive than pristine natural grain. I frequently put them in with no prompting from natural flaws because they are ever so much more attractive than bland wood.

Here is a stairway where I have begun to create such faux flaws using both paint and carving. I have to credit my border collie pup Scotty with most of the fine work on the left corner. He shows a natural enthusiasm for wood carving, though most of his student work is so overdone that the pieces are basically disintegrated. We are working toward a more moderate style. He has shown such diligence in leather stropping his cutting tools that the entire family is pretty nearly barefooted!

Click here for higher quality, full size image

From contributor U:
If I'm not mistaken, I believe that there's something about CO2 that pulls out the bubbles, so either drag a torch across it at a very low setting, or blow on it with your mouth.

From contributor D:
Systems Three T-88 epoxy is the product that the questioner referred to. This is a gap-filling epoxy. It does not cure to a brittle film, so it won't crack. However, it is not water white. The T-88 won't develop air pockets because of its long open time and even longer cure time. Its open time is about 40 minutes at around 70 deg F (longer, if colder). Its cure time is 12+ hours, also at around 70 deg F (longer, if it's colder). You can use fiberglass resin or surfboard resin (uses MEKP as a hardener). These could develop air pockets in them. The best product to use is Konig's Rex Lithe. This is a clear polyester gel that comes in a squeeze-out tube, like toothpaste. It mixes with their hardener (I think that this is also an MEKP hardener). In an hour you are ready to sand and continue.

From the original questioner:
Thank you for pointing me to shellac sticks. It's about time I tried that. And I love the way Scotty "ages" your work.

Contributor J, thanks for clearing that up. I thought it was heat that popped the bubbles, by melting it or something.

Contributor D, thanks for the tip on Konig's Rex Lithe. Does it stay clear after you put the hardener in?

From contributor E:
West System epoxy with graphite powder. Should be available in any marine store or through West Marine, Hamilton Marine, or Jamestown Distributors. The powder turns the epoxy a nice, deep black. Perfect for inclusions in any color wood.

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor A:
I think the problem with grain filler is that all grain filler is formulated for shallow fill only and cannot be used as a deep fill. That property allows you to easily remove the excess, for one thing. The West Systems Epoxy will work there as a deep fill.

As to the bubble question, the bubbles are usually caused by exothermal chemical polymerizing. So the slower the epoxy to harden, the fewer bubbles you'll find because the heat generated is very small unless you are filling or tinting the epoxy. Then the filler slows the process, allows heat absorption, contains the gas and you don't have much bubbling.