After playing around with this hydrophobic fumed silicon (Cab-o-Sil
TS-720) I've changed the way I look at this substance. It can be used as
a "fly-drying" agent, which is how I've been using it, and it works very
well for that, but it's also a "waterproofing" agent. The treated fly
floats better than the dry untreated fly. That's because hydrophobic
(water-resisting) particles embedded in the fly repel water molecules.

To get maximum floating performance I'd do this:

1. Treat the fly before fishing it, while it's dry. Immerse the fly in
fumed silicon, twirl it around, rub the particles in, and blow away the
excess. Repeat.

2. After the fly gets wet use a stiff brush to push the substance into
the fly. Use a lot, to excess. The brush that comes in a Frog's Fanny
bottle works great. The substance will push the water to the surface
where it can be shaken off with false casts. Repeat.

A few points:

1. You'll use a lot of the agent this way, so for this to be economical
you need cheap treated fumed silicon, not Frog's Fanny or its imitators.

2. If you want a low-floating fly don't use treated fumed silicon on a
heavily hackled fly.

3. This stuff changes the status quo for tying dry flies. You can tie
very sparse patterns with hydrophilic (water attracting) materials and
still get the fly to float if it traps fumed silicon particles. It could
even be incorporated into recipes. For example, you could mix it into
the dubbing and tie it into the body.

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