present challanges. 1. Mid to late summer usually will have the presence of monsoon storms. 2. Also, these months will create a very hot substrate (surface), which won't stop the painting, but I would recommend avoiding this scenario if you have that option. If the ambient temperature is 105*, then the surface temp could be 140*+? To answer the question 'Is it too hot to paint an exterior in May?' . . . May and September will also fall into this category, but once in a while Mother Nature will change her mind. Maybe not worth the risk. Donny Fargo