Fiberglass,JB Weld and other two part when activatied create on heat source once combined from their chemical reaction.For example mix part A with part B then you will feel a heat develop.The tempature around the product can effect the curing times of said product.
When it is hot outside I will use less cream in my body filler mix and when it is cold outside I use a little more.
Long explanation short....JB Weld is a two part mix that hardens from a chemical reaction of the two parts.The enviromental tempature around JBWeld will either slow it down or speed up the curing process.
Hope this helps
Oh ,I noticed you are getting fibergalss(resign)..remember the more liquid activator you add to the resign the less work time you have before the resign begins to cure and harden.