Enamel Matt Varnish

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.

you also might want to try spraying closer to the model or at a lower pressure if using an airbrush.  Or closer to the model and use short burts to keep it from running if from a spray can.  Sometimes the matt can dry a bit before it even hits the body and cause that.  When it happened to me it almost looked like sea water that had evaporated and left the salt there in spots..