Here in Southern New Jersey we've got about 25 small airports and plenty of planes that need to be cleaned. Yes, there is big money in cleaning planes, however don't be fooled into thinking it's easy money. It's not. It's hard work. Hey, I'd much rather be doing a dockside maintenance detailing of a small boat.
What's it like to detail a small plane? I've been detailing planes for more the 15 years and I can tell you first hand that it's not much fun, especially when it comes to cleaning the underside of a plane. You know, where all of the grease, oil and engine exhaust dump out and stick onto the fuselage of the body of the plane during flight. It's also not much fun cleaning, polishing and waxing the underside of the wings on a single engine plane. It's also not much fun working around all of those metal rivets.
Now-a-days I am very selective about the planes I will work on. Just like working on garage kept cars, there is a very big advantage of working on planes that are stored in hangers. Those types of planes stay in a lot better condition for a longer period of time that those left outdoors. The other thing is, you can't be in a hurry to get the job done quickly. I do it mostly as fill-in work. That way I can take my time and do the job right.
Since I do everything by hand, I really have no competiton. That allows me to price my work way above most plane detailers. When I first started doing planes my prices were $600 - $700. Now there well over $1500 for most small planes.
From a marketing standpoint, doing planes is like doing boats, you almost always get to also detail the owners vehicles too. And, like Gary said, you also get plenty of referrals of other plane owners. If you like hard work, it's a win, win situation.