I'm considering putting a circuit board in a ventilated NEMA 4 enclosure outdoors. I'm a little concerned about humidity and condensation inside the enclosure. Do I need to conformal coat my circuit boards, or take any other precautions?
Requirements depend on environment,but, yes, conformal coating is, at a minimum, a really really good idea. Apart from condensation and misc stuff that manages to get in, ants can wreak havoc, depending on ventillation holes other denizens may also do their bit.
If you want a 'quick and easy' coating material that is a lot better than nothing at all then clear polyurethane "varnish:" spray is useful.
There are many commercial products offered for this purpose. Dow Corning make some good ones but there are many others as well. (My only association with Dow Corning is as a satisfied customer).
Here is a useful tutorial page from Dow Corning. While it obviously steers you towards their products it also provides good general advice.