Like could you put down 2-3 coats of black epoxy primer, a few coats of grey filler primer,
and then use a clear coat over it, to see how light reflections move over the car?

I'm finding that I'm using the reflections of lights from the clear coat, to see where
irregularities are. Also, it would be easy during sanding to see where the glossy
clear coat has not been sanded off, indicating a low point.

Or is this not recommended?

not recommended,,,to thick.
Just use water if you want a shine....

Just use a guide coat to see the irregularities while sanding.

For a temporary 'shine' just spray some w+g remover on but don't let it sit there evaporating,wipe it off asap

JCCLARK wrote:Just use a guide coat to see the irregularities while sanding.

Agreed. That's why they make the stuff.