“Foursquare” is the best word to use in describing how a spray gun should be held relative to the surface that you are currently painting – any deviation of angle is likely to cause faults in the finished paint, possible ones that are large enough to merit stripping and starting over again. Although the human hand is not an absolutely precise instrument, and spray gun techniques are robust enough to accept a little “wobble” in the angle, you should try to maintain correct positioning as accurately as possible throughout the paint job.
The spray gun should be aligned with the panel so that it is spraying straight and direct at the surface – head-on, one might say, rather than slantwise or on a diagonal. Depending on the angle of the panel, this may require some acrobatics on the painter’s part to achieve. Never hesitate to use a stepladder, kneel, sit, or even lie to achieve a good painting angle. It is better to clamber up on a stepladder and get a good angle than to remain on the floor and end up ruining the paint job.
The spray gun should also not be tilted up or down relative to the metal surface. Any kind of angled spraying will produce too-thick paint at one end of the pattern and too-thin at the other end. It is for this reason that the spray gun must be kept perpendicular to the surface in all planes, so that the paint will adhere to the surface in a smooth, even, precisely-laid-down coat.
Furthermore, you must be careful to avoid arcing, which is swinging the spray gun in a curve rather than moving it in a straight line parallel to the surface and maintaining the same distance throughout the stroke. Arcing is, unfortunately, the natural motion of the human arm, so it is necessary to consciously train yourself to avoid it when you are beginning to paint.
Arcing is bad for a good paint job because a heavy layer of paint will build up at the center of the arc, where the spray gun swings closest to the surface, and the paint grows thinner and thinner towards the ends of the arc, since the spray gun is further and further away. Larger drops are also likely to strike, raising the risk of orange peel considerably.