Cab-o-sil is based on silica, which is sand, which is decomposed rock. Wood flour, of course is wood-based. If you use cab-o-sil, you're basically making an artificial rock. If you use wood flour you're making artificial wood. And if you use cell-o-fill, you're making artificial cardboard.

With all of these, the final characteristics are determined by the epoxy/filler ratio. With a rich mixture, the characteristics are closer to a glob of epoxy. With a lean mixture the fillers start to take over.

If you need rock-like characteristics you use cab-o-sil. If you need wood-like characteristics you use wood flour. And if you just need a cheap thickener it's cell-o-fill.

Rocks are hard, brittle, difficult to sand and bright white. Wood is softer, more flexible, easier to sand and a closer match to the unfinished boat color. Depending on which characteristic is most important to the job, that's the filler to use.

Happy New Year.

Laszlo

As noted the filler material is composed of different material.

Epoxy is very heavy and dense but clear and can be very brittle.. To make the fillets lighter, one can add a filler to the epoxy mix to make it lighter and more flexible. The filler or flour will displace epoxy with a lighter material and also cause the epoxy to encase the filler with a honeycomb like lattice of epoxy. This structure will be more flexible than solid epoxy. It also makes sanding easier.

The choice of wood flour, saw dust, or cell-o-fill can depend upon where the material is being placed and how much weight one wants to eliminate. The filler can also color the epoxy. For visible sections like the cockpit one can use woo flour to make the fillets appear closer to wood and maybe more visually appealing.

West Systems Filler

Some concrete applications add fiberglass fibers to the mix to provide extra strength to the concrete.

Thanks Laszlo & George, that's exactly what I needed to know.

I don't think that I can disagree with any of the replies, and I have only one boat under my belt... so far!

Cab-o-Sil (colloidal silica) is used for high strength bonding... e.g. if you were bonding something that was going to have a winch bolted to it, use something like Cab-O-Sil instead of wood flour. You should use a respirator to keep it out of your lungs.

Wood flour (fine sawdust) is cheap and makes a great thickener for fillets. As well as being easier to sand, wood flour is a lot cheaper than epoxy. I have a large tub of wood flour... probably a lifetime supply.

I have Cab-o-Sil, but also have a large tub of Cell-o-Fill. It is cellulose based, but I would also use it where high strength is needed. However, if the joint needed to be REALLY strong, I would probably use the Cab-O-Sil just because I have it. The Cell-o-Fill is just considered a nuisance dust. Try not to breath it if you can, but I would use a respirator if I thought that i wasa going to be getting a lot of it in the air before mixing. However, it's a LOT less hazardous than Cab-o-Sil. I would also use a respirator when using graphite. However, if I'm mising outside and there's a breeze, I just use caution and stand up wind. I have a large tub of Cell-o-Fill... probably a lifetime supply.

Micro-Balloons are light and easy to sand, so are great for fairing a hull... when not finished bright.

Keith