Quilly,
Living in the same type of humidity problems you have, I have finally solved my problem. I've tried the silica packs, and the grease. Both were a pain in the arse, especially the grease. I have a reinforced concrete vault that I built myself, that is integrated with the basement walls and floor of my workshop. Needless to say, the humidity/condensation is quite a problem, especially in the summer. Last year I bought a "goldenrod" from Cabela's. It cost about $30. I'm sure you've seen these things in the catalog. They work well. It plugs into a regular wall outlet, didn't raise my electric bill that I can tell. Supposedly runs for about 5 cents a day. No rust at all since I've been using it, about a year now. It gets warm, but not a fire hazzard, you can grab right on to it without burning your hand. It just warms that space about 2 degrees warmer than the room it is in, therefore driving the moisture out into the room.

They are rated for cubic feet, I bought the one that was one size "too big,", but I think that works better.

Good luck, MH