I have heard that an additive like Floetrol should be used when spraying exterior paint. Is that true?
Flotrol can be used for exterior spraying. It offers some useful benefits that water alone can't do. Slows down the drying of paint a bit; this can allow you to get a smoother finish when needed, good for spraying exterior doors. Can reduce flashing that can occur while spraying on hot days and using dark colors.
Using Flotol can also cause runs or sags.
Normally, when spraying siding or other large areas this additive isn't needed. Adding Flotrol to your paint depends on what you need from the finish and if you need extra time to work the paint, like back-rolling or back-brushing.