paint gun help

i bought some new guns the other day so i can paint my interior of my truck along with some other things...i got all the materials and whatnot. well i went out to spay my plastic dash pieces with some flex promoter stuff with the gun....this is my first time ever painting with a paint gun so im trying to teach myself along the way so far so good but i had a question, when the promoter is coming out of the gun, it blotches on the plastic pieces.. or it dries im assuming fish scally....is this normal? also, there is a air adjusting screw at the bottom where the chuck is, but if that is there then what is the point of having the regulator? could someone please explain those adjustments to me and maybe some painting tips? the directions for the gun were practically worthless so thats why im here. thanks in advance.

oh i just read somewhere that it will come out blotchy on the hvlp guns if the air pressure is too low, which i think it was...the shop i bought the guns told me to run them at like 20 psi....but i just read where guys are runnin anywhere from 40-45 psi...it makes sense. more pressure would blow the promoter faster and finer. not these big ol' drops like i have going on..lol. but if you have any other recommendations, they would still be helpful.

All I know is primer and base sprays around 30 psi, and clear around 45-50. Its kind of personal preference from there. Thats just a starting point really.

cool, thats a big help because the flex additive is similar to clear coat. i have to sand the few pieces down again and give it another try. lol.