its not absolutely necesarry, but the 60 degrees is recomended. it will cure at colder temperatures, but much slower. you would be surprised how much a small change in tempereature can affect cure times. i live in texas, and i was using some smooth cast 300Q which has about a 1 min working time before it gels. i had 20-30 secs. i would recommend using extra hardener beyond what it says.