I took ours out completely, washed the insides with bathroom mold and mildew killer, then left it on the top of our central heating boiler for a few days until it was completey dried out (glass side up, hole side down).
When it was completely dry, I ran a bead of clear silicone sealant around the joint.
When this cured, i then turned the rubber ring on the sealing cap around, smearing with vasoline on both sides before reassembly.
This was about 3 months ago.. it hasn't fogged up yet.
Incidentally, it now looks cleaner than the other side.