Filling Big Gaps in Walnut Burl

Thoughts on ways to fill cracks and knotholes in Walnut burls before applying a clear finish. March 4, 2009

I have 200 or so walnut slabs that I have been making up into gallery furniture. I have my water based final spray finishes dialed in, but I have not been able to figure out a good filler for gaps up to 1/8" and multiple holes. Ideally I would like to trowel it on, sand it off, and then put down a coat of thinned poly to pop the finish, then spray, so I need a filler that has: good gap filling qualities, can be darkened with pigments, sands well, minimal shrinkage, and will take a light color coat (BLO, oil based wiping poly). Does anyone have any ideas?

Forum Responses
(Finishing Forum)
From contributor N:
A few are Epoxy, West System, or System Three. Call these guys up and have them recommend a transparent filler to add to the epoxy resin and its hardener.

From contributor R:
Why not apply some blue tape on each side of the crack, split, and fix the missing eyes. Get yourself some white Bondo or a product similar to Bondo –it could even be a clear Epoxy. Dab a bit of Van Dyke Brown or Raw Umber colorant on a piece of cardboard and judiciously add a bit of the color to the filler; mix your hardener into it and trowel it into the void. It will only take a few minutes to harden up - but remove the tape before its totally hardened. If you wait too long to remove the tape you might just pull everything right off the surface.

From contributor H:
I have done this several time with good results. Use fiberglass resin. I have also used clear adhesive for solid surface material. If you don't fabricate solid surface then you don't have the gun or the mixing tips.

From contributor A:
Most epoxy companies recommend waiting approximately seven days before top coating. Many products (definitely oil based paints/varnish) will not cure on top of fresh epoxy resin. Target makes a trowelable waterbased grain filler. Give them a call and see if it will fill 1/8" gaps without cracking.

From contributor T:
I would recommend System Three Sculptwood or Quik-wood type epoxy putty stick. Either can be tinted with dry pigments. They aren’t trowelable but can be pressed into place with a credit card type plastic coated with a little soap to get a near perfect fill.

From contributor C:
Ureaformaldehyde - Unibond 800 with the dark catalyst. Add sawdust for bigger gaps. Anyone else using this? I find it works well on blown out knots. It has a fairly low viscosity, so it needs some masking tape to hold it in place while it cures. With sawdust in it, it is spreadable like epoxy.

From contributor P:
I am with an adhesive distributor and we distribute a knot filling system that looks great and allows for mass knot filling because of the ease of use. It's a technology from Europe and you literally push the product through a glue gun. You can "fill" or "coat" knots very quickly. I currently have three large door manufacturers "coating" knots in mass to secure loose pieces and for aesthetics. Since it goes through a glue gun, you can switch different "needle" nozzles for small knots/cracks or larger orifices for large holes.