Dan,
I have SEM high build primer that I was planning to use for this same application.
(
https://www.semproducts.com/product/high-build-primer-surfacer/42013)
Calling that stuff "high build" is a bad joke at consumer expense. Per the datasheet, it is less than 10% solids by weight. Compare with PPG K36 for example, at more than 50%. You would spray a whole case of cans to get equal build.
Can't prove it, but I'd also bet adhesion is better with the two-part catalyst activated professional product.
Does the darker primer have to be epoxy based? Or would any dark primer suffice?
I use dark epoxy primer under a lighter high build for two reasons. First, while block sanding for surface flatness, I can see when the high spots get thin. Second, if sprayed "wet-on-wet", the epoxy primer ensures superior long term adhesion.
"Wet on wet" means spraying a cross coat of epoxy primer, allowing the solvent to flash off, then spraying two cross coats of high build, roughly 4 mils. Although the solvent has mostly evaporated, the epoxy is not cured, and the two materials bond. For K36, PPG says the primer
must be overcoated with 7 days. Per a good long conversation with a PPG rep some years ago, I wait about 30 minutes. The rep's description was "about as long as it takes to clean the gun and mix the high-build".
A 4 mil wet coat will shrink to about 2 mils when dry. You can block it to a half mil or a little less over the high spots, then spray two more cross coats and repeat. Note two rounds nets about 5 mils of build in the low spots when the highs are at minimum, which tells a lot about how close you want to get with your original surface contouring using micro.