mottle

hi guys, if you get mottling then say you clear the car....how can you determine what actually caused the mottling? striping seems straight forward, but how do you determine if you applied the base too dry, too wet, maybe the air pressure wasn't right, etc? When I first started out I'd spray...

I sprayed a friends car a solid color called hyper blue (subaru) and in the sun I'm seeing slight dark spots almost like mottling. Not exactly sure what caused it, but could wrong temp reducer play a part? I don't think I sprayed the base too dry or anything like that, but I'm stumped on this...