Rub a tiny amount of patch lube around the OUTSIDE of the nipple before you slide on the cap, waterproofs it nicely. For my flintlocks (or flinch rocks, hehe) I put a thin bead of grease around the outside of the pan and in an arc over the touch hole in the barrel before closing the lock. Seals 'er up nice and tidy.
Enjoy the heck outa camp...after two years missing muzzleloader deer camp, we are resurrecting better than ever here in the Black Hills!
Good hunting!