This warning can be found on almost every paint can you'll ever pick up or it should. Under no circumstances can you paint in direct sunlight. Plan your work so that you can follow or stay ahead of the rising sun as it moves through the day. I would say in front of that way it doesn't have a chance to heat up the surface you're trying to paint. However, this is not always possible. The biggest reason not to paint in direct sunlight is that the vehicle namely water in latex paints will evaporate out of the paint too quickly not allowing the other chemicals in the paint to properly penetrate into the surface and create a bond.

Latex paint is a surface build product as it is with limited perforation. It doesn't matter if the product happens to be oil or a stain the same rules apply. This will only cut the coatings durability and protection causing it to fail, costing you more money. Just because you apply paint to a hot surface and it dries almost before your eyes, doesn't mean it's ready for a second coat in two minutes as some painters would think. Painting over a coating that is not fully dry or remotely cured will also lead to failure. This problem is called solvent entrapment, and unfortunately, too many people practice it. People that don't bother to follow the manufacturers' instructions. Always read the labels of any coating you are working with for the best possible use of the product and for your own safety.