The professional auto body repairman knows that, before you can apply paint, the car must be totally clean and free of debris. This process involves washing and drying of the vehicle and making sure the paint booth is completely clean without any air movement at all. Event eh slightest breeze can blow in unwanted particles into the wet paint and cause blemishes. Many professionals invest tens of thousands of dollars into building a special room that they can paint cars without having to worry about the elements.
We use rollers and some Rustoleum aluminum paint cut with mineral spirits in my dusty garage. At 150 feet you could never tell the difference between our paint job and a professional one...make that 150 yards.