It’s always best to wax your car in the shade, out of the direct sunlight. When your paint’s finish gets hot, it “bakes” the wax as it’s applied, hardening it to a point where it can be difficult to remove. If you have no other option but waxing in the sun, then use a synthetic wax. But remember, the less sun, the better – try working in the early morning or late afternoon when the sun is at a low angle.