Mark,
I use the dry powder silicon type floatants for really small flies. With the Frogs Fanny you need to jamb the powder into the hackles and body. When it comes in contact with moisture it forms gas bubbles that help keep the fly afloat. The gel type floatants can actually sink a really small fly.
For most of my dry flies, I use a gel or liquid type floatant. Loon makes a good product as does Gink.
For really large dry flies, I have found Mucilin to be a great products and a number of my guides use it as well.
None of the floatants will work for a long time. There are a number of factors that cause a fly to sink. Foam from the top of the water, dirty leaders and tippets, fish slime from the fish you just caught, choppy water, wind action across the top of the water, hydraulics of the water pulling the fly and leader down...they all factor in.
Larry ---sagefisher---
Organizations and clubs I belong to:
Fly Fishers International Life Member
FFI 1000 Stewards member
FFI Presidents Club
FFI Fly Tying Group Life Member
Washington State Council FFI
V.P. Membership
Alpine Fly Fishers Club
President & Newsletter Editor--The Dead Drift
North Idaho Fly Casters club