Epoxy does yellow, it depends on which type you use as to how fast it will Yellow.. I use the Zpoxy 5 minute a good deal of the time.. it will yellow, but not as fast a number of other brand 5 minute types.. I have some Baja baitfish patterns that were epoxied 6 years ago and there is some yellowing.. but not much. I always coat the epoxy afterwards with Hard as Hull nowadays... Just be sure to allow at least 24 hours from the time you apply the 5 minute epoxy until you coat it with the Hard as hull, so the complete Curing process can do it's thing. the 5 minute is the Set up time, not the curing time
If you'd like to avoid the extra step of coating the Epoxy after it has cured, Purchase the Zpoxy 30 minute Type epoxy. This is some of the Strongest epoxy available.. It has a set time of 30 minutes, which means a rotary wheel is a Must have tool.. (I use the rotary wheel anytime I apply epoxy, even with the 5 minute type). the advantages of the 30 Minute type are numerous: It is Waterproof (5 minute isn't)
; it is very strong once cured; It does yellow, but at a much slower rate.. almost unnoticeable (or at least the fly will be demolished before you'd notice any severe yellowing. the 30 minute epoxy seems to bond better onto the fly in the long run as well..
Also you can cause the Set up time to speed up by mixing abit more the Hardener then the resin when doing a batch. a common mistake is to mix up too much epoxy for the Job. I did this mistake a number of times years ago, until I just started mixing up less.. a good rule of thumb is to make a drop of the Resin part and then next to it a drop of the Hardener part. look them over before mixing to make sure they are even, thought I always have the mind set that having abit more Hardener then resin is acceptable, Not the other way around
... I often will have to mix another small batch because i didn't make enough for the first batch.. but this is better then mixing up far too much. Also figure you will only be able to coat maybe 4 Baitfish heads before the Epoxy reaches it's time when it is no longer viscous enough to apply smoothly...
a little practice and a little patience and you'll be "smooth sailing" with Epoxy in no time... It is a valuable material for your flytying....