Wojtek is right Dan. Basically, three things can cause this effect;
1.) Paint too thick.
2.) Too heavy a coat, jet too close, successive coat applied immediately after or during application of first coat, combination of all of these.
3.) 'Fall out' or 'splash back' from shading or protruding surface, eg., tailplane diverting paint onto the vertical surfaces or vice versa.
A VERY LIGHT rub down will remove this on enamels. A gentle but progressive rub with a soft cloth should cure the problem with acrylics.