Jorge,
Get a can of black fast drying spray paint and mist it over your primer. Hold the can about a foot and a half from the surface and spray when there's no wind. That's your guide coat. Then block sand until all the guide coat is gone. Sanding through a guide coat helps to assure you've sanded the entire surface. It also makes sure you've sanded it smooth because you can see it in the low spots even after you've sanded off the high spots.
You can use a DA or random orbital sander, but the swirls may show up in your top coat. You could try that on the bottom and see how it turns out. That cobalt blue is going to be a lot less forgiving than the white. Dark colors will show flaws in the prep work much more than light colors do.
Dave