Spray it real good with Scotch Guard!
I used to do that to my fletching to keep those feathers from soaking up rain water.
<br>
<br>Seriously, change primer often, cover the lock ( a cut off leg from a rain coat is what I have used ), and hold it with the lock under your arm, and hope. Some times nothing will work. I missed a big Turkey Gobbler with my percussion rifle once. There was enough dew fall and humidity to screw up the cap. Since then I keep my caper under my jacket and change caps often. Flinters are even more vunrable, as you well know.