Cadflyer, If your spraying Omni paint which is a product of PPG paint it is Ok to spray when there is humidity or rain present.The old laquer and thinner paint systems is were the problem in painting with humidity,it caused the paint to "blush,a whitening of the paint when appplied.I presme your using omni basecoat (color)clearcoat.I usually spray at about 35 lbs with an HVLP gun(high volume low pressure)but it dosn't matter your paint won't blush,and the type of gun doesn"t matter ,Spray painting in the rain actually is better with omni or any other urethane paint is good because it keeps down the dust.I have a body shop and been spraying 25 yrs.(cough,cough).Ihope this helps you good luck. Carl