First of all, welcome to AF, and more specifically, the "Car Modeling" section of AF.
What the person above me is trying to say is that for the most part, the "Car Modeling" section of AF mostly deals with plastic, resin, and white metal modeling. It rarely deals with Die Cast as a medium, but some people here will do some of that as well. If you start here
http://www.automotiveforums.com/vbul...play.php?f=926 you will find the link to the Die Cast section. It is also very likely that this thread will be moved to that section once a mod sees it.
To answer you question... I'm not an expert, but I think you should be able to paint the primer right over the red spray. However, the more layers you put on, the more paint build up you end up with, and the less detail you end up with. It is normally easiest to go right down to the base medium again, in this case, die cast metal. However, there are better ways to strip the paint than sanding, such as dipping it in Castrol Super Clean, or Easy Off Oven Cleaner, or Brake Fluid, or professional Paint Stripper (careful on this if you have any plastic pieces to strip paint off of). You can read more about stripping paint in the FAQ (see my signature) or by doing a search.
Good luck!