What causes "toothpick streaks" when spray painting already-painted metal?
Something is causing the layers of paint to repel away from the areas, leaving a significantly thinner coat.  There are three phenomena at work.  


surface contaminants, such as oil, wax, silicones, etc.
A glossy surface, which after all is designed to prevent dirt and contaminants from sticking.  
fundamental differences in the adhesion or absorption of paint due to different surfaces.   


You cure that with a three step process: 


First, you sand it sufficiently to remove gloss, rust and contaminants like silicones.   
Second,  you wipe down with a strong enough solvent to remove oils, waxes, and the like.  This solvent must not leave residue.   A two-cloth cleaning is best to assure you're not just swirling the contaminants around. 
Third, you use primer to form a surface that is consistent in regards to absorbing paint and adhesion, if those issues exist.