Jason...try a diluted
APC solution and a brush....not to stiff and not to light....a mediun...
First hit it with a jet stream of water to dislodge most grime...then spray and brush it...hit again with a jet stream...you should get 98% of gunk off....
I know some have used a dye on the like Ritz frabic dye...2.99 a bottle and mixed in spray bottle with water....worked good..nice and black again...and no harm ....
AL