I've never had trouble primering in the garage when it was raining outside. When it gets cold, I do bring the minis inside to dry instead of leaving them out there. Also, I always keep the spray cans indoors. I'm in Portland, OR, btw.
We actually always primer in our garage.. We've been talking about building some kind of booth for .. well, years, since we moved in really, but have never gotten around to it. 6 years so far and it hasn't killed me..