First of all you dont need water for rust. Its a catalyst and a bloody good one thats all.

Secondly who said you cant rely on oil leaking in there??? Ill keep the neg rep to myself but its called PCV... not a leak.

And i would suggest that because of the PCV system that mild steel pipe (most cheap stuff has no coating) would rust through from the outside 100 times before the inside would.

Paint the outside, dont bother with the inside unless you really like the smell of spray paint... Must keep glasses on, prevent ppl seeing eyes, am high as kite now...