Time is your enemy, and also your friend here.

The garage may be shady, but it can also trap heat. You may have to just wait for a good day when the humidity is low to do some of the work.

A slow cure hardener can extend pot life of your resin on hot days. I had none, so I just mixed small batches -- frequently!! I went through a lot of plastic cups and stirring rods. A trip to the grocery store supplied the cups. Broken and scrap cedar strips provided the stirring rods.

Coat the first side of the wood, either with epoxy (to seal it), or with the fiberglass and epoxy, on any day you can work it. Once one side of the wood is sealed you want to leave the other side open until the wood is as dry as you can reasonably expect it to get. This may mean waiting a few days (or many days) for the humidity to drop. Check local weather forecasts and when a dry, sunny day is forecast, pull the parts out into the sun. Let them sit there all day so the sun's heat and the dry air help evaporate residual moisture in the wood, and then cover the second side late in the day when the wood is good and dry and the temperature is dropping.

The idea is that once one surface is sealed, moisture can only escape from the unsealed side. Since you would normally coat the outside on one day, and theinside on another, this timetable is reasonable.

Accept that this is the best you can do, and either decide to live with the results, or you can fuss over the few problem areas that MIGHT develop. There is a good chance that you won't have any problems.

If you have the opportunity to do some of the work inside, you might consider cutting the parts and sealing them with epoxy in your basement. Then bring them outside for sanding and assembly. This should make them far less likely to be affected by outdoor humidity during construction.

The downside of this is that your panels may get a bit stiffer and heavier from the extra epoxy, and of course you have to pay for the epoxy, so there may be a slight increase in cost. This might be a difference of a quart or so of resin, which would weigh about 2-3 pounds. Not a big increase in weight or cost. On the plus side, coating the precut pieces means you might need a bit less resin when you apply the fiberglass. Since the wood is sealed, excess resin won't soak in, and could cause some runs.

Any cuts you make in the wood after it is coated will allow moisture from the air to enter, but the amount of moisture that can get in is going to be very slight. Hope this helps. Paul Jacobson