I finally got hold of my powder coater and asked him about the idea of primer. He said 99.999% of the time there is no need. Some less reputable coaters avoid the expensive sandblast-to-white-metal step and dip the parts in a phosphoric acid etch solution hoping for a good bond of the melted plastic.
There are proprietary industrial plastic coatings that 'require' precoating with a phenolic epoxy plastic and final coating of the outer surface. Funny thing, California outlawed the phenolic epoxy primer and the vendor issued a product bulletin that said, "Shazam, we just discovered that the outer coating works great without the primer so keep buying our stuff!".
My guy started getting questions from new customers, "Do you metal spray before powder coating?" Strange request until he found out that some guy in northern California was doing that. Not a good idea. Although metal spray does have a VERY rough and porous surface and would bond extremely well to the plastic, it sometimes does not bond well to the base metal and flakes off. Also it would at least double the cost of the final job with no significant improvement in performance. Metal spray is old tech as far as decorative coatings go.
Finally, my guy says that for some hard to cover colors, he will 'prime' with a similar easy to cover color, then put on the final color.
For our purposes, just take your grungy parts to the coater and ask for their standard blast & coat service.