The tackyness acts as a binder allowing a second coat to adhere to the first after it has been cured. I like this because some flies like surf candies I tie in steps. For me it is definitely a benefit, a little clear nail polish and you are good to go.
I've been reading up on these UY polymers. There are two types, acrylate radical polymers and epoxy cationic polymers. The fly tying resins are acrylate polymers.
The failure to cure is due to the chemistry of the acrylic polymer. Acrylate polymers cure by free radical polymerization which is started and maintained by the UV light. However, with free radical polymerization, the exposed surface of the polymer allows the free radical to bind with atmospheric oxygen to create a peroxy radical that slows the polymerization at the surface. This is called oxygen inhibition. Oxygen inhibition slows down the polymerization process, and the result is peroxy radicals on the surface that are incompletely polymerized.
If the polymerization is done in the absence of oxygen like under the surface, or in a nitrogen atmosphere, or in a vacuum, it would all polymerize. Oxygen inhibition is NOT so a second coat can be put on. It is a failure of the polymerizations due either to the wrong frequency of UV radiation, or the amount of radiation, or both. Lack of cure in my opinion is not an advantage for most fly tiers. I can put on several coats of my resin and the coats adhere to each other.
The solution to a tacky surface is not to add a second coat that also fails to polymerize. The solution is to modify the chemistry of the acrylic polymer so that it is resistant to oxygen inhibition. There are such polymers and there are research papers on ways to minimize oxygen inhibition.
When I did my research into these polymers I found the problem was NOT finding an acrylic polymer that would resist oxygen inhibition. That was really the easy part. You can find them, but they are expensive especially in small quantities if you can even get them.
The problem is finding an acrylic polymer that is cheap enough to be used in fly tying, that has enough oxygen inhibition resistance to cure tack free with the thicknesses used in fly tying, that can be purchased in small amounts, and that can be completely cured with the hand held low power UV flashlights. It is not easy; and I have bought, and thrown away many acrylic polymers that either fail to cure at all with the lights we use, or that cure with a tacky or greasy surface. I eventually found one that is cheap enough and cures tack free with a flashlight.
For those of you that use a UV polymer that leaves a tacky surface, if you give enough time and UV exposure, these sticky polymers will eventually polymerize. Put the flies out is the sun should cure the polymer.
