Krylon Fusion paint is made specifically for plastic.

 

I used a Krylon primer with "fusion technology" for the first primer layer.

I've used some colors of Fusion before on other items that tended to stain (permanently) when exposed to oil. That's not desirable for a valve cover, especially the one with the oil fill tube in it. I'm also not sure the temperature limits on Krylon Fusion.

As I posted above, I stacked primers, first the Krylon that I know bonds to plastic well, and the Duplicolor "engine paint" primer over that, before a color coat and a clear coat of the Duplicolor engine paint.

Stacking primer like that is usually a good choice if you aren't sure the primer that matches the color coat will bond to the material underneath. One manufacturer's primer will almost always bond to another manufacturer's primer. In theory any manufacturer's paint should bond to any other manufacturer's primer, but that doesn't always work out.

Duplicolor said that the engine paint primer "should bond" to plastic engine parts like ours, but when pressed further, they said that Duplicolor engine primer would definitely bond to the Krylon primer if I wanted to go that way, and that Krylon primer specifically designed for plastic might bond to the plastic parts better than the engine primer.

I gave up on the white letters, and painted everything metallic gold. I need to take some pictures now. The valve covers are in, along with the intake pieces. I haven't installed the new front cover yet, and I am actually looking for a real metal front cover that has the mounting holes for the crank position sensor. If I can find a metal cover, I'll probably use that, and paint it to match with the engine paint.