I would look for the type of wire coming into the fixture, and whether the wire entry is sealed also. Some fixtures use a paper wrapped 3-conductor wire. The fixture can be completely sealed, but water can wick up the paper, causing the fixture to have water or moisture inside. If this is the case, completely seal the wire entry into the fixture, but also seal the supply side of the wire, so that no moisture enters.
Another solution, is to use large silica gel packs. The same kind you get with your shoes or electronics, just a lot bigger! These will absorb any moisture that gets in the fixture, and stop it from condensating (Is that a word?) on the lens. If the fixture is flooding..these won't help, but if it's just atmospheric moisture, they should do the trick...just don't eat them! lol
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