What did you spray it with, airbrush or rattlecan?
The mottled effect sounds like uneven mixture of the flating agent (actually a pigment) in the paint. This is common in rattlecans, especially when they get old, or you spray cold. It can happen with an airbrush, especially with very thin mixtures, or if the flating agent has flocculated (clumped) slightly.
The only other thing I can think of is a very uneven spray pattern from your airbrush.