For the ‘cheapies’ among us:
Silicone Water Guard, available at WalMart in the camping section, will permanently waterproof your flies. After that, you don’t ‘need’ anything else but a clean and dry fly and if it’s tied correctly, it will ‘float’ forever.
But dry flies ‘sink’ for all kinds of reasons.
Drag: If the tippet sinks (flourocarbon sinks) or the current pulls on the tippet/line the fly can be pulled under.
Bouyancy:
If you don’t have enough bouyancy to float the hook, the fly will sink regardless of what you do to it. If you are counting on the surface tension to hold up the fly, like many traditional dries, you have to make sure that the weight of all the materials doesn’t overcome the surface tension. Real bugs weigh less than even the lightest wire hooks…
Fish slime:
Fish slime is a pure wetting agent, if you don’t remove it the fly will sink unless it’s made with foam or cork, etc.,. Your floatants, shakes, sprays, whatever won’t help. You have to clean the fly or it will sink. A bit of dish soap and/or a vigorous wash in the water can remove this.
Waterlogging:
Even if you pretreat the fly, if you tied it with dubbing or any loose material like that, water can get into the spaces left between the fibers and add weight to the fly. If enough water weight exists, the fly sinks. A samadou patch or one of the super absorbent synthetic cloths can help suck the water from a fly and allow it to float better. Best is to just let the fly dry out (change flies).
Be careful with the ‘dry shakes’ type stuff. What most of them are made from is a super absorbing powder that takes in water. After use, if you don’t get it ALL off the fly, when the fly next hits the water, the remaining powder will try to absorb the whole river, sinking your fly very quickly. Ten or more very fast false casts will sometimes flick of the remaining stuff, but doing this without using the dry shake will often do just as well.
And, lastly:
Rain-X is WAX, not a water repellent. It adds weight and won’t help your fly float. What makes it repell water on GLASS, is the ‘polishing it off after it dries’ on GLASS. It’s a very fine wax that fills the micropores in glass so that it becomes slippery. In itself, it ‘repells’ nothing. The ‘film’ that you polish off of the glass won’t help your flies float, actually the reverse. Since you can’t polish the flies after you add the Rain-X, and since flies are not made of glass, it’s just more weight.
Buddy