NOT A PAINTER

Demonracer said

One of the reasons that the paint chips away is that the valve covers are used. Being that aluminum has porosity, They have absorbed the oil mist under the hood. Powder coaters bake aluminum pieces to burn off this residual oil. I blasted some valve covers, soaked them with acetone and let them air dry for 5 days. I then baked them in a toaster oven at 200 degrees before painting and sanding. They turned out very well and still look god 12 years later.

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I agree. Demonracer has a valid point.
And I will add this comment. If you read the tech sheets of several or so auto paint co. they can comment on the use of self etching primers, that is the use of a certain primer (like epoxy) on that before any topcoat. The self etch has an acid and that enters into the equation. But I rather dislike Dipli Color for decades. You would be surprised to find some laquer derived spray bombs out there. Best bet is always a topcoat mixed at the auto paint store and a $30 HF gun. Costs more but fewer issues.