The best rule of thumb is that you should never do ANY detailing in direct sunlight. The temperature on the vehicle surface will always be hotter than what you are currently feeling. Products that have a liquid components to them, will evaporate quicker than intended and can streak, dry up, scratch the paint (if you are using a compound and it dries up, it will swirl the paint like crazy).
Specifically speaking about PolySeal, PolySeal has been reported by a few of our testers, to leave streaks when applied to a hot surface. The solution was to go back over the PolySeal with a glaze and it buffed right out. At the end of the day though, Xtreme Solutions does not recommend applying any products in direct sunlight if avoidable. There are always exceptions to the rules and solutions to problems that arise, but if you can avoid it – avoid it.