Spray Painting - Blushing/Milkiness
There are so many attributes that go into spray painting.
An outcome of one of these attributes being missed is Blishing/Milking.
This is what happens when you paint something and then air moisture condenses on the paint films surface which results in a milky-grey blemish in the paint. This can occur straight away or after a little while.
To prevent this you can paint when humidity is low and leave your painted pieces to dry in a warm area.
You can read more about "Common Car Paint Problems" at PaintCars.com