You might try a product from Balsa USA called Magic Primer. http://shop.balsausa.com/product_p/454.htm Works great and can be topped with just about any paint you want. This product is used on full scale A/C covering using the Poly Fiber (Stitts) process. It should be thinned 3:1 with Poly Fiber reducer or REAL MEK if you can find it.

Good luck