BBB, powder coating goes on dry using an electrostatic charge, and is then liquified in the oven, generally at around 400F and then cools to form a rigid coating. I believe that as your springs flex the coating will crack allowing moisture underneath. If that happens you'll get corrosion beneath the coating but it won't blister like paint does.

I believe that you'd be better served to clean your springs, spray a good coating of epoxy primer, and then spray them with the color you want. Paint is much more flexible than powder, IMO.