rust on my msd distributor

I just spent a lot of $ building a new engine ,and I found a lot of rust on my msd distributor . I was wondering if this is a common problem ,how it got the moisture , how many of you had the same problem ? and is it effecting your spark or causing you issue"s ? I hate to ruin a new motor over having rust in my distributor throwing a disruption or bad spark , or is this common for msd distributors . my 340 ran ok with it ,but with a new motor do I want to put it back in ?