I used to work in printed circuit board fabrication, so let me add a couple of things to Alastair Heaslett's "fix" for moisture in pwb's. The fix, as he describes is fundamentally:
1) get moisture out
2) try to keep it from getting back in
Additional points:
a) when cleaning, be sure to remove any excess soldering residue between traces or pads. (Rationale - rosin provides a place/pathway for moisture to accumulate
b) after baking, let the board cool in a dry atmosphere (e.g. a sealed container with desiccant in it. (Rationale - to prevent moisture from the air being re-absorbed, while the board cools)
c) when coating the board, be sure to coat the edges and the insides of any holes, which are bare. (rationale - to prevent moisture from "wicking" along the edges of glass fibers, which are exposed on the edges, including holes) We would often coat the edges of pwb's with the same epoxy used for solder-mask.