How do you clean it? Huh? Yea, how can you clean undercoating, or can you?

The factory undercoating on my 68 Charger is 40 years old of course (actually it's 43 now) and obviously it's showing wear and it's dirty, as in road grime dirt. I've spent a few hours so far scraping off the loose coating but what I can't figure out how to deal with the dirt that's stuck to what remains. My plan is to recoat the areas where it was scraped with new but I don't want a bunch of new black patches next to the dirty brown spots. I tried a wire brush and that did next to nothing. Would a wet, stiff-bristled brush work you think? I wouldn't think spraying new coating on top of dirty old coating would be a good idea. Ideally pressure washing it might do the trick but that's just not practical in my situation.

Before you tell me to heat it up and scrape it all off just know that ain't going to happen. I like the stuff and I want it to stay.