Epoxy users question

Can plain old wood sawdust be used as a substitute for collodial silica, for a thickener ?

Jack L

Epoxy filler
Yes---- However, the intended use of the epoxy mix should be considered.

Epoxy mixed with wood flour is ideal for “filling” tasks such as small gaps between strips on a stripper hull under construction. It has the major advantage of being easily sanded. This mix does tend to sag while curing if used in vertical applications.

Epoxy mixed with with colloidal silica is stronger, harder and sags less. It is also much more difficult to sand.

If you are considering this epoxy mix for stitch and glue construction, use wood flour (sandable) with part of the filler being silica (to reduce sagging).

Contact West Systems for expert information.


Wood flour is the same thing
You can buy wood flour if you don’t have sawdust already.

It’s a bulking agent for the epoxy, with the advantage of allowing you to adjust viscosity. Too little flour = runny; too much flour = dry (don’t do this).

Silica has other properties besides bulking. My epoxy manual (System Three) described all the ways epoxy could be used, and the various agents used in combination. Maybe it is online at their website? That might give you the best answer to your question.


I’ve done it with OK results but the wood flour filler is cheap and easier to use because it is fine.

The trial kit at Duckworks is a deal you may want to check out:


Rougher surface Jack …
If just trying to bulk something up go for it … If you sand the epoxy / sawdust fillet down, you should probably re-coat it with neat epoxy if gonna be wet a lot.

FWIW, the ashes from your BBQ pit make a nice hard filler too.

Just Did That
Jack, I have a Japanese saw, double blade w/fine and a courser blade. I’m doing a scarf on a gunwale and used the sawdust from the cut, using a jig, to mix in with G-Flex. Worked just fine. From using the fine edge of the saw the dust was very fine in texture and worked with the resin. I used 220 sandpaper to knock off any high spots and then ran some 0000 steel wool over that before hitting it up with Watco. Came out nice!