I have used both candle wax or Burts Bees lip balm to seal a pan in the rain. I remove the lock, put a bead on inside lock around the pan where it mates with the barrel. After installing lock I put a bead around rim of pan, then after priming, I seal the junction between frizzen and barrel.
I have hunted all day in a downpour and my flinters still fire using this method. I do a lot of still hunting, and find it difficult to move through the woods quietly negotiating brush and deadfalls while keeping the lock tucked under an arm or coat. I do use a cows knee as well, but find they can work in a drizzle, but in a full out rain, moisture gets into the prime without sealing the pan.
The last deer I killed with a flintlock was on a very wet day, with a frizzen sealed with Burts Bees lip balm, which contains bees wax.