I have posted this before. I did two '32 Fords that were originally shot with DP40 primer. within a couple years they were starting to look bad!

Primer was never meant to survive exposed to weather. Cheap paints of any kind will fail early because the pigments and resins are "cheap"! You can't expect it to last!

I shot the first '32 in a "semigloss" mix of catalyzed black urethane, and PPG DX685 flattener. (This flattener can also be used in clear without producing white specks.) It was hard to get an even finish on this one. Wherever it went on a little wetter, like in corners, it dried a little shinier.

The second one I did the same thing, but used a "full flat" mixing ratio. It was really nice, and still looks great 10 years later.