Hi there karoliens when useing a airbush the directions should gey you close. But not always right. You did not mention wich primer it is oil base or arcrylic?
But that rough Finnish is normally caused by letting your pants land on the surfaces to dry.
When this happens it's not a wet enough coat to flow out.
Fixing this is possible by holding it closer to the model to get that wet look.
But other factors can benefit also such as air pressure the mix of the paint to thinner ratios and depending on all those how fast you move the airbrush across the surface. And keep a eye on where your over spray is going so it's not landing elsewhere on your model. If it looks dry while your spraying that's how it's going to look when dry. If it looks wet and shinny when spraying that's probably how it will look when dry. I'm not trying to confuse you but to wet = paint runs.
There has probably been more volumes printed about this than any other topic of modeling.
It gets down to how you do it to make it work. Find somthing to practice on is your best bet. It's trial and error and practice is you best bet.
It's not that hard you will get past this in no time. I hope this helps you with your projects. Welcome to modeling and have fun with it.