NOT A PAINTER

That sucks, looks good from a distance.
Did you wipe them down with a pre paint cleaner?

I agree with media blasting them, it will rough up the surface and give the primer something to stick to. Die-cast is way too smooth of a surface, even with self etching primer, then wipe down with self said pre paint cleaner.
Clear coat always turns yellowish cloudy on me after a while you may want to use a heat lamp to cure it.
And I would try using a piece of sandpaper glued to a block of wood instead of a sander.

When I paint, I use products all from the same manufacturer, cleaner, primer and paint. The only thing I hate more than painting is repainting after it doesn't turn out well the first time.

Didn't even know there was special primer for aluminum.