The article I read was a bit vague on whether an additional coat of primer was needed, but the author did use one to check up on additional bodywork he did.
As long as you're careful with the paint application, I don't think thickness will be that big of an issue. In fact, to get the best results, your color coat will have to be thick enough to withstand some sanding and polishing... Just in case, I've included a link about painting plastic cars; I'd assume it'd be the same for diecast once the primer is applied.
http://www.briansmodelcars.com/tutorials/tutorial.asp?TutorialID=12&CurPage=1
(Alex also has his published at his own site, http://italianhorses.net)