> Can anyone tell me a little about thixotropic agents. I laid up a hull
> yesterday and for some reason, most of the resin ran into the bottom. I
> can't figure out why since it never did it before (I've laid up about 15
> boats without this happening).

> What are some of the agents and what are the advantages and disadvantages
> of the different agents?

> Thanks! Dave

The old standby is Cabosil, a trade name for fumed (or aerated) silica. Aerosil is another trade name for the same stuff. Under the microscope these particles look like snowflakes, and the jagged edges lock into each other when mixed with resin and prevent it from running. It's very light stuff. 1 cu. ft. of cement weighs 94 lb., but 2 cu. ft. of this stuff weigh in at 14 lb. A 2 cu. ft. bag costs $88 at TAP Plastics around here, or you can buy in smaller quantities and get gouged. The stuff works so well, and is so cheap, that it hardly pays to look for something better. But there are other alternatives.

Common cooking flour will do in a pinch. Sanding dust from the dust collector of a power sander works not nearly as well as Cabosil, but it's free and matches the color of the wood. It darkens more than the native wood though, so it's best to mix Cabosil with resin to the desired consistency, and color it with sanding dust until it matches. Common cement dust works ok too, but the gray color puts some people off. There is a product called ViscoFil which works almost as well as Cabosil but costs more. Chopped fiberglass strands are sometimes used, but they get all fuzzy. Hollow phenolic and glass microspheres are sometimes used to lower the density and ease the sanding of fillets, but they are not thixotropes. Load the resin with Cabosil first, then mix in the microspheres. Sanding dust from epoxy works too, but characteristics depend on the grit size of the abrasive, and density is high. If I left anything out, someone please jump in.

Watch out when working with any of these products. They fly around, and are easy to breathe. In your lungs they have the same effect as asbestos. Wear at least a dust mask to head off problems. The respirator you should be wearing with epoxy will work just fine with the thixotropes.