For those who can afford $0.50 to $0.70 per day during the most humid months (and far less in our drier months), the addition of a dehumidifier will spare you much of this agony. It won't prevent condensation if you open the garage door and your cast iron is cooler than the outdoor air, but even then, once you close the door again the dehumidifier will quickly pull the excess moisture out of the air. It will also provide you with a buffer against condensation if you quickly open the garage door (then promptly close it) since it takes some time to exchange the dry air in your shop with the moist air outdoors.
I can not tell you how grateful I am for my dehumdifier. Before I added it to my shop I had to polish all my cast iron surfaces pretty much every other week during humid months. Since adding the dehumidifier several years ago i only polish the surfaces about once per year -- and even after that much time it takes very little effort to polish as there is very little rust to remove (rust is pretty much limited to where sweat has made contact with the cast iron). The same goes for all your iron and steel hand tools and bits -- the dehumidifier pretty much eliminates rusting tools.
I know the extra $15-$20/month in electricity will be too much for some hobbyists, but for those for whom it isn't a major hardship will find it an investment worth making -- not to mention a major savings in labor, meaning more time to spend doing actual woodworking. You also get a much more comfortable working environment as well since sweat actually evaporates at 45%RH, and your wood pile and projects will be much more stable without the wild humidity swings, greatly simplifying layout and assembly.
For a typical 2-car garage sized shop you will want to consider a dehumidifier rated at 50-70 pints/day capacity. Additionally, the operating efficiency of dehumidifiers increases as the capacity increases, so it is generally worth investing in a dehumidifier a size larger than you expect to need. You can also add an external pump to discharge the dehumidifier's condensate out a nearby window, or elevated drain, if a floor-level drain isn't readily available -- sparing you the added chore of regularly emptying the condensate bucket.
Just my thoughts...YMMV!
