Questions about paint

Posted in Custom Workstation

I am using model master's acrylic paint and I've been trying to paint some of my figures. I try to start off with using white paint as a primer. I believe its official name is "white gloss". Anyways, I have to end up painting the figure with 5 or 6 coats, and even then it still looks streaky and doesn't completely cover the figure. Its as if the paint is too thin. When I use the black paint, it takes about two coats, three at most, and the figure looks TOTALLY BLACK.

Because of this, I have just been priming with black. The problem is that the white paint still won't paint over the black. I figured this was just because it was a lighter color over a darker one, but the yellow paint I have goes over the black just fine.

So whats up with this? Did I buy the wrong color white? Do I need some kind of thickening agent?