Cropdusting has been my absolute dream job since I was really young. I grew up on a farm and observed them all the time-back in the time before GPS. Like any career, in talking with the pilots, it has its up and downs-some love it, some hate it. Soon I am transfering to NAS Lemoore (I'm in the Navy) to the middle of farm central in CA.
Have you been to the airshow at Oshkosh? They actually have forums to teach people how to do what you love doing. Airbrushing is a very fine art, and yes, there is a market for it. If you have a chance, check it out, attend a forum, and network. If not, advertise and see if you get any bites. You do not need any license to do that type of work (unless you are using poly). Make some examples of your work, though. If I had the talent, I would. However, with my plane, I just tell everyone it is a 15 foot paint job-looks good from 15 feet
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Good luck!
Wittmandriver