Painting PVC material

This should be an easy one for those who have done it and it worked out ok.

PVC is great! Sintra, Komatex... love it! 10mm & 13mm have all but replaced plywood in my shop... but I do still have customers that need black, royal blue, red, etc. backgrounds on their signs. Colors are readily available in 3mm, and limited 6mm thicknesses. Our attempts at finding a paint that will hold up on PVC over time have not been real sucessful. Laytex... forget it. Standard enamels won't adhear well, and too hot of a thinner can damage the surface. Plastics paint (Lacryl, etc.) tends to fade with direct sun exposure, don't cover well and have little to no gloss. Automotive paints and hardners seem to be the best choice we've found. What advice can you give?