Even a high quality catayzed primer was never meant to be exposed to the sun.

I repainted a couple locally built '32 Fords that were shot with a PPG epoxy primer, and in about 3 years they were fading. By the five year mark they were medium gray with white streaks running down the sides. The primer was breaking down due to exposure.

You probably aren't going to find any primer that will last very long. The only permanent "fix" is a topcoat black with flattener added. A catalyzed paint is even better.