Big problem is waste disposal.
when you're doing it for yourself at home, you can squeak through getting rid of the waste on clean-up day, taking in on haz-mat amnesty days, etc..But once you start doing it commercially, the rules applicable to a business hit you.  Safety-Kleen man becomes your best buddy, but unfortunately one visit from him costs more than you made from last twenty parkerizing jobs.
That assumes that you do it all on the up and up.  By the way, EPA makes ATF look warm and friendly.