Temporary paintjob on dart swinger - matte grey?

Unless it's an epoxy primer it will be hydroscopic, so any areas where it is over bare metal will rust through in short order.

Most of the cars you see these days with a matte finish are painted with a base coat of your color choice then covered with matte clear. You get all the protection of a base/clear with out the hassles of trying to flatten a single stage.