What I've read seems to corroborate with what Scooper has said, that the flex of the frame will cause cracks in a clear coat which will lead to rusty spots. Brompton offers a clear powdercoat of their frames which looks rather handsome. I wonder if that's more durable than on a full-size frame as the folding frame might not flex as much?
I intend on experimenting with intentionally rusting and preserving with shellac on my next Trek project, a 510 with Reynolds 501. (the paint's already too trashed to recover). Much in the same way user MeatloafOvadose has done to their GT over on the Dropbar MTB thread. This is nothing I'd try to mess with on a frame with thinner tubing, that's for sure. I figure that as long as I make sure to apply frame saver to the inside of the tubes, any rust that forms on the outside can be managed.