well , yesterday i was in my companies office , and we have a "demolition man" pinball there
i looked at the coindoor , and now know exactly what you mean with the effect you are discribing
it looks like they put very fine structure through the paint
like it had a bad sprayhead when they painted it, so it looks like some spickels of paint that are in there that are thicker than the rest of the paintjob
this is not the hammerite effect i think, now i saw a close up of the coindoor on that pinball.

this is the effect of hammerite, and that is not what you are looking for i think.
this is the effect you are looking for i guess:

maybe you can mix some rice that has been chopped up in a
blender? hihihi

or maybe some very delicate layer of sand between 2 or more coats of paint .
just experience with some piece of metal i would say

goodluck, and keep us posted !