Good Black Priming

When doing a black primer over miniatures you have to first assure yourself that you have cleaned the miniatures of the unwanted casting elements and after that, that you have cleaned with water and liquid soap or something similar the miniature. The primer is needed so the later paints will stick easily and last on your miniature. Sometimes, although I found it quite often, the priming process can skip some parts of the miniature, especially if it is a detailed one, with many ridges. An easy way to figure out which parts of the miniature are not painted is to look at the miniature in a shadow, (employing artificial light, if needed). The white metal unpainted spots will pop-up immediately, so you can fix them. Otherwise, when starting adding the basecoat paint, you may be surprised that it does not cover that good the miniature and this may be one of the reasons why. 

In the image below you can easily see the not primed spots on those ships.

I advise priming only those spots, as another full priming of the miniature will cover the details of the miniature, a.k.a. destroy the miniature, especially in the case of microminiatures.