There will always be rainy days. Unfortunately in the windshield repair business, water and resin do not mix. Simply wiping the water from the surface is not sufficient. When it rains, water seeps into a break on a windshield and if you try to repair it, the resin will not completely setup and cure. When the water eventually evaporates from the repair, you will be left with ugly air pockets inside of your repair and an unhappy customer.
If you have an indoor facility, then you can dry off the windshield with a towel and use a hairdryer to evaporate the water from the break before the repair. This will take some time. You want to make sure that the glass does not get too hot as this can cause the break to crack. Or you can simply let the water evaporate naturally. Whichever method you choose, take your time and make sure that the break is free from moisture before proceeding with your repair.
If you don’t have a facility to do an indoor repair, then simply postpone it until the next dry day. If you explain to your customer that rain can ruin the repair, they will gladly wait.