It can be done, but it takes lots of patience and practice.
My dad is a master at brush painting but he's been doing it for about 40 years. I don't know if I'll ever hold a candle to him.
For me, the toughest part is to not get too much paint on the brush so that you get either brush marks, or lumps.
I've been out of the gig for a while, so what I do to practice is go buy up a bunch of small cheap planes (like 1/144 scale) and practice camo patterns and stuff on them. I also have a failed 1/72nd B-24 liberator that is used as a practice platform as well.