bp man,
Let's sit down a minute and think this through.........5,4,3,2,1. OK, now deep breath and blow it out!
First off, LOSE the WD40...worst stuff in the world for any firearm, got it??? Now, if there is a humidity problem with just your muzzleloader and just the muzzleloader, then it must be one of two problems. One: You're not getting it clean sufficiently (ie. a good hot water and soap bore flushing) and leaving behind a residue of black powder or bp substitute or reason #TWO: Your preservative is NG and not displacing the water as it should. Try Ballistol as a final pass through for light cleanings. Can be used on everything including the stock and leather strap too.
I'm a firm believer in storing my firearms after a good cleaning using Vasoline Petroleum Jelly or RIG......nothing better!!!!!!! Yes it is a petroleum product, but when you want to get ready for the next "season", just clean the bore with a good solvent like #9 Hoppes or my favorite is "Ed's Red". Then de-grease the bore with Windex or rubbing alcohol before firing a couple percussion caps to lightly clean the bore and firepath and you are ready to load for the first round. I spit patch each and every time between reloads and there you have it. Keep it Simple........ and you won't go wrong!
Jim