Dred the Red

Are you painting on bare plastic?

What type of paint ( enamel or acrylic ) are you using?

i usually paint the piece flat white before using any of the primary colors,it seems to help. If you have and airbrush do the same first and the color will be brilliant when dry

I'd second the advise of a base coat of white.  Also, remember, Red is in the same tricky catagory as yellow, and white, etc.  Depending on the brand of paint, acrylic or enamels..the results can vary more.  My personal rule of thumb is..if I intend to paint a large area, I'll airbrush it over a base coat of white.  If the area is small, or if I'm detailing, I'll usually use a #4/0 spotter.  At any rate, a white base coat will help the Reds, Whites, Yellows, etc..cover fast and more opaque with less layers to apply.  Good luck!