Some advice on paint effects please?

I’ve been trying to make a good Two Lane Blacktop replica Chevy (maybe more of a tribute, as the '57 in the game is quite different from the '55 in the movie), as I haven’t managed to find a good one to download (if you know of a good one that isn’t just a grey paint job, please let me know and I’ll download it instead and abandon this and go back to driving). So far, take a '57 Chevy, throw on the cowl induction hood, find some wheels that will fit the bill in the absence of the proper American Racing 200-S (I’ve gone with Torq-Thrusts so far), paint the whole thing a dark primer grey, and we’re almost there. Here’s where I need some advice.

As you can see, the primer on the movie car was very aged looking, with lots of streaking and blotching (side note-IRL, people recreating this paint job spray a little paint thinner on uncured, still drying primer to get it to run a bit). I’m trying to figure out how to replicate this effect, and everything I try looks kinda terrible. So, if any of you uber-talented painters has any advice on how to pull this off it would be greatly appreciated. Or, if anyone would like to just make a good one I (and a few others I’m sure) will happily just download that, as I’m not much of a painter anyway.
Thanks!

I tried doing a matte grey paint on a car, then adding very narrow paint splats in different grey colors and translucences and layering them, but I couldn’t get it looking any good either…

I’ve been working on this off and on since FM4. This is apparently a tough one. I’m obviously not a great painter, and stuff like this (and even just slapping some numbers on a car and having it look passable, to be honest) makes me respect the work of the good Forza painters (such as yourself) even more. Thanks for taking the time to try!

no problem- I’ll keep giving it a shot and seeing if I can find something I like.