Surface prep, it's all about...
I know this thread is some months old now, but revisiting this question with some finishing people leaves it still an open issue for me. We know that poor surface prep sets up an immediate recipe for the failure mode turtle mentions -- nice smooth powder coat over rust, that's only evident when a chunk of it falls off or is chipped off. We really don't want any rust on our tube structure, and we want to see it if there is any.
Apparently the preferred pretreatment for steel is a zinc phosphate conversion coating such as MIL-DTL-16232G Type Z Class 3 or 4. This is a multi-step hot process typically done in a plating shop. How do we know that a "powder coated" airframe or engine mount has been processed this way before coating?