the only time ive had moisture issues is after a shot has been fired...burnt powder residue draws moisture like no other...even on a mildly humid day, your pan will look wet after its been burnt pretty quick...
today, my buddy couldnt get his gun to fire..i could see the sparks coming off the frizzen pretty good...i had him look in the pan and sure enough, a big ole gooey ball of priming powder! he shot earlier and reprimed..thats when disaster sets in...
my tip, carry patches soaked in rubbing 91% alcohol..clean the frizzen, pan and surrounding areas..make sure you get the bottom of the frizzen, the face of it, and the pan clean...blow on it lightly to get it to dry if its real cold out..alcohol dries with no residue and dries very quickly on its own (i like to blow mine dry when its cold) you could dry it with a clean patch as well...alcohol also removes oils and anything else...
i carry a small pill bottom with alcohol soaked patches and another with dry patches...i clean my lock with them and swab the bore with them when im at the range or need to swab for whatever reason...
alot of guys say i carry too much in my possibles bag...but when their gun isnt going off, or their flint shatters, or they run out of loads, or need a ball pulled etc etc etc, they run to me coz they know i got it lol...
never messed with anything to keep my powder dry other than a leather cows knee...and knock on wood, never had trouble with it...
i believe 2 and 3fg granules are actually somewhat coated from the factory so they dont draw moisture...4f isnt so it will ignite easier i believe...could be wrong but i think i read that somewhere...maybe prime with 3f on wet days....ive primed with it and it works about as well as 4f if you got good spark..