Tallow is rendered fat- like lard, but it could come from beef or venison, too. You could just as well use lard, so long as it's not the salted variety.
I agree with Ken on the 2f. I'm in a really wet climate (120" of rain a year), and it seems like the courser the powder in the pan, the better it resists humidity. I've seen no penalty on dry days for using 2f rather than 3f either.
As for that big ole rain drop. Did it happen to come from the brim of your hat? Been there, done that.
I agree that the gap between the pan cover and the barrel is the real stinker in serious rain. If you don't have that covered/sealed, you're going to have wet prime. I use a cows knee in the rain as a matter of course, but I also run a little bead over the joint between the pan cover and barrel to prevent big drops like you experienced once the cows knee is off.
In my case I've been using an olive oil/beeswax mix (7/1), but it could just as well include fat. What I like about that particular mix, body heat is enough to soften it almost instantly into a wet paste. Hold a little ball of it in the palm of one hand briefly till it softens, then use one finger to smear a little over the barrel/cover joint. It doesn't take much and you have to redo it if you refresh the prime. But it's so easy, I do it as a matter of course.
