Water is sometimes deterimental especially in oil based paint shops. Air line is the primary source of water in a spry booth. Water should be completely drained off from the compressor more frequently especially in PU (polyurethane) system of painting.
The presence of water can be traced by using water fining paste. This can be applied over a dummy metal and a painter can apply air using the spray gun on the metal sheet applied with water finding paste. If water is present in the air line it is clearly made visible with some discolouration on the pasted area.