OK, I see we did not do a very good job describing how dew forms in the garage. We have already had phone calls and emails saying "What do you mean dew in the garage? How could dew form in the garage?"
Let's use the word "condensation" rather than dew and see if we can make this make sense.
If we take two pizza pans out of the oven, one a metal sheet, and the other a Pyrex sheet, we know that the pizza will still be warm 30 minutes latter on the Pyrex sheet, because glass stores the heat much better than metal.
Now the same theory works in reverse. If you have a cold mirror in the garage, it takes a very long time for it to "warm up" along with morning's rising ambient temperature.
If you take a 45° F beer out of the garage refrigerator, it immediately gets covered in condensation. Guess what happens when you take a mirror that cooled down to 45° during the night and expose it to the morning air? Yes, the same condensation.
Why doesn't the secondary mirror dew up in the morning? Because the secondary mirror is a relatively small amount of glass, so it quickly keeps up with the rising morning temperature.
The primary is a heavy (sometimes 200 pound) chunk of glass, it takes a long time to cool and a long time to warm up. This is often called the "Flywheel Effect".
Let's recap: The Primary mirror becomes very cold in the garage during the night. As the morning temperatures rise, the Primary lags far behind; still cold from the night. Condensation forms on the cold mirror in the morning hours. The condensation mixes with dust and pollution on the surface of the mirror and becomes acidic. The acid eats through the incredibly thin aluminum coatings.
Many customers have never seen this condensation, because it has evaporated by the late afternoon. If you wanted to see it, check the primary around 10am.
How to prevent this from occurring:
Place a 15w lightbulb, dew chaser, or even reptile heater on the floor of the rocker box - under the primary mirror. This will keep the Primary just barely warm enough to not form condensation.
If you put the dew chaser on a timer (let's say from 4am to 11am), it will heat the mirror during the dewy morning hours, but let the mirror cool down for quick use in the evening (and save some electricity).
At a star party, aim the mirror cell East when you tie down for the night. The rising sun's rays will help warm the mirror from behind. If you have any portable 12v fans, aim their airflow at the face of the mirror.
If you still get some condensation, rinse with distilled water.