I often use an enamel auto primer (i.e. Krylon, etc.) in the typically large rattle cans found in most any department store hardware or automotive section. These are typically meant for, you guessed it, metal auto bodies. Never had a problem with such a primer having a bad effect on plastic of any sort. Reason I mention this is Tamiya paint is expensive and comes in small cans.

In any event, should you decide to use this more economical method, be sure to test spray the primer on some scrap plastic to be certain the paint does not craze the plastic.

For a top coat, most people often seal the paint and decals with something like Testors Dullcoat Lacquer. There are all kinds and brands of dullcoat or clear matte finish. Tamiya makes some too if you just have to use that brand.

-Harq