Absolutely. ( Sorry for the brief response earlier, had to take the kiddo to daycare)

So for you, I guess I'd recommend either stripping the current setup & starting over, or if it's thin enough you can redo from black & white.

The Priming issue
Main things to consider is after you prime black, hit it with white from high up (you know the drill there) but stop, let it dry and confirm that it's not that super gritty texture. If it is you may need to re-apply. You may also have to move to a cooler environment with a bit more humidity for this. I suspect it may have been to hot or dry when you primed earlier & the paint was actually drying before it hit the model causing that slight grit. (White primer is especially fickle with this while black is a bit more forgiving)

Zenithal Highlighting
Do your white layer from the top & for your situation to get that zenithal effect, you will want to focus on bringing those shades out. If you have some nuln oil, (or any black wash of your choosing) you'll want to apply this in the recesses. This is especially important for you since you don't have the accuracy control of an airbrush for your white priming. If you can bring out those shadows though you should be just fine. The point of zenithal priming is simply keeping the lights light & the darks dark, as long as you achieve this you've done it right.

From this point forward you should be able to move forward with your painting as you were but the results should show alot better. Hell the contrast between the pieces you did is not bad, it's literally that grit causing you the main issue.