Cab-o-Sil -- which type to use?

I called the Cabot Corporation, which manufactures Cab-o-Sil, to get
information about their products. They were amazed that fly fishermen
were using this stuff for drying flies, and were kind enough to send me
samples of both hydrophobic and hydrophilic products to test. (I now
have several lifetimes' supply -- about two quarts of each.)

The two kinds of Cab-o-Sil that I tested we

M-5 untreated fumed silica, hydrophobic

TS-720 treated fumed silica, hydrophilic

The result in a nutshell is that the treated, hydrophobic TS-720 product
is the one to use. The untreated, hydrophilic M-5 is worse than useless.
It turns into a gooey mess when wet and causes the fly to sink. The
TS-720 works great. After I dried a completely soaked fly with it, the
fly floated better than even a fresh, untreated, completely dry fly.

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