Ok matey boy its time for some air brush 101 .. first thing is dose your compressor have pressure control valve on it ,,if it dose then turn that down to like 20psi ,less is more here.
I will take longer to spray dew to the lack of pressure but the results speck for themselves .
On the back of your brush you can remove the back tube,thats where the needle lives, if thats to open you will get a large amount of paint,close it till the tip sticks out of the head but do it genitally as the metal is pretty soft ,also there is a screw in front of the trigger ,you can adjust that to get the exact amount of spray you want .
Takes some farting around but once your sorted its heaven mate .
Spraying free hand ,there's nothing wrong with it ..many AFV crews resprayed their vehicles in the field so no two were alike but once you have your brush adjusted you will get the fine lines your wanting , an old Iwata i used to have could spray lines nearly as thin as a pencil lead until 35kgs of high grade steel flattened it ,,bugga!!
oh well
Phill