I've always had that trouble, so these days I try to limit the amount of water that gets to the fletchings ( i know, imposible really!
)
-I tried fishing fly dry liquid stuff and it was rubbish.
-I tried spray on silicon spray (fabsil gold I think it was), it worked but was quite heavy.
- I tried hairspray (somebody suggested it). rubish.
Now I use woodsmoke! I found that by waving the fletchings through a very smoky fire the formaldehyde and other things help to coat the feathers and protect them form moisture
I keep meaning to try cold smoking a bunch before fletching the arrows, but keep forgetting
NB, I found it does discolour white feathers, but since I normally shoot pink it's not a problem.
(REAL men shoot pink btw
)