Cell-o-fill and woodflour are both thickeners (thixotropic agents, if you want to get fancy). They allow the epoxy to take a shape and stay in place, rather than dripping and running. Both are essentially chopped fibers and contribute no tensile strength at all since the fibers are so short (essentially dust).They also contribute negligible shear strength. Their main strength contribution is in compression, but it's not really much compared to epoxy's native compressive strength.

Cell-o-fill is chopped cellulose (paper), woodflour is chopped lignin (wood). That's why cell-o-fill is cheaper. My personal preference is for woodflour since it's a better match  mechanically, chemically and visually to the wood the rest of the boat is made of, but that's just a preference. If all I had was cell-o-fill, I'd be OK using that for any glassed fillet, tape or sheet, since it's the long glass fibers that are responsible for the tensile strength and control the shear forces.

Laszlo

   Thanks, that's all very interesting. The only reference I've ever consulted for this topic are the CLC manuals and, though they don't state this outright, the implication seems to be that somehow the cel o fil is somehow stronger. And when I say 'implication' I really mean 'interpretation' by the reader. 

So when you say 'better match mechanically and chemically', you don't mean 'better' in the sense of a quantifiably stronger bond, right? 

   I can't prove it, but for some reason I think cell-o-fill thickened epoxy is a little harder to sand than wood flour thickened.  A not that I like sanding epoxy at all, but that's a very necessary evil with these boats! :)

Adding the "useful" comment I decided to make - for "next time" when you're creating filets that will be visible/bright/varnish finished: I carefully mix in wood flour and cell-o-fill together in proportions, such that the epoxy paste almost exactly matches the color of the okume.  Highly recommended by me to avoid the dark zebra stripe effect that wood-flour-only filets create. 

And if I'm creating filets in areas where I have stained the wood  I add alcohol based stain to the filet paste to get a near color match. (A little goes a long way...)

   Thanks Bubble, I've always used all wood flour on any fillets to be finished bright. And yes, it does look stripey. I hadn't considered lightening w/some cel o fil. 

Right now I'm on fillets that will be covered up with paint, so color is moot.

   I think the cel-o-fil makes a smoother putty especially when making thicker mix like fillets.  It is harder to sand than wood flour filled mix, but not as hard as the Cab-O-Sil fumed silica powder that they used to use in the kits.  The silica is a lung hazard so really needs a respirator when handled, so cel-o-fil is safer.  So, I think I'd use up the cel-o-fil, maybe with a bit of wood to color it like said above.  If you are careful to get smoothly tooled fillets when still uncured, there won't be much sanding anyway.

Creekboater wrote: So when you say 'better match mechanically and chemically', you don't mean 'better' in the sense of a quantifiably stronger bond, right? 

Correct. I wasn't talking about the bond, I was talking about the fillet itself. Woodflour means that it uses the same chemistry as the plywood - lignin and epoxy - so it will react to solvents, paints, etc. the same way the plywood will. By mechanical I meant things like the coefficient of thermal expansion. As the hull cycles from cold to hot and back again, the fillets and the plywood will expand and contract in similar ways.

Laszlo

Laszlo and Bubblehead make useful comments about the differences between cellofil and wood flour.

I'll add that I think of them much as I do the aggregates used for making concrete: large aggregate adds compressive strength while also increasing volume; smaller stuff just fills in the smaller spaces in between the big bits. Cement is the expensive binding agent as is epoxy; additives change the consistency of the final mix and how it behaves both during placement as well as once cured besides improving the performance of the bonding element.

Wood flour adds stiffness to uncured epoxy, making placement less of an issue when it's not being used to form a matrix with reinforcing fabrics.

Adding cellofil alters the character a little, making the resulting mix easier to smooth before cure begins, perhaps a bit harder to sand as well owing to its shorter, more consistent fiber length.

They both have their uses for us when it comes to modifying how epoxy behaves during building then in its performance and longevity over time.