Louis,
Kurt, what kind of desiccant? A bit more detail here? How well does it work in a downpour?
Last question first. The answer is kinda like it is for black powder hunters or poker players. Just keep the stuff "close to the vest and hunkered over." Nothing works in a real downpour though, including trout.:)
I had the good fortune of being informed years ago that Seidel powder was being discontinued, so I bought several cases of the stuff. Any of you remember the flat tan squeeze bottles it came in? I transfer it into a film canister to facilitate stirring around in aggressively with the fly locked in forceps. Something else I never see mentioned - Don't put dirty flies in the stuff. You'll foul the whole batch in no time. For the same reason, don't pour the stuff in your hand, grind the fly in it, and put the remainder back in the bottle as some do. Even with a clean fly, the oils and dirt from your hand and fingers will quickly foul the stuff as well. You could throw away the excess, but a bottle won't last long that way.
There are basically three types of silica desiccant, the key difference being how the stuff is processed:
1. Granulated (The older Courtland brand. It's horrible stuff)
2. Powdered (the old Seidel and several other brands. The good stuff. It has the density to use enough pressure to force the water out)
3. Fumed (Frog Fanny & others, great stuff for finishing but not dense enough to really get in and dry out a soaked fly, which is the first step. There's so little resitance, it's like swishing around in a cloud)
Even a good desiccant won't work for long if the fly is still wet at the core. What happens is it repels for the first few casts, but once this surface coating is removed, you're back to casting a wet fly again. Simply putting a wet fly in the canister and shaking won't get the job done. You have to go through the steps I mentioned in the other thread. I use a good cotton hanky to absorb the moisture between steps because I always have them around. A good sized piece of amadou is the ultimate for absorbing moisture but is usually too thick to stroke the hackles and squeeze the body separately between the fingers on small flies.
Finally, never, ever mix using desiccant with floatants, i.e. pastes, greases, oils, creams, silicones, sprays, or what have you. You have to go one direction or the other. Sorry for repeating myself on this last point, but I see so many guys still doing it, and then blaming the desiccant for not working properly. Pre-treating with a non-oily Scotchguard type of silicone works very well and won't conflict with the desiccant if done far enough ahead of time to be thoroughly dried & cured before use.
BTY - you can buy the stuff on line in bulk for cheap. When my supply of Seidel runs out, that's what I will do.