'68 B'cuda inner (plastic) grill detailing tips?

looking for help with cleaning/detailing my '68 barracuda inner plastic grill.
-what product worked with out damaging them and tool used, etc.

I have one that has what looks like road tar or possibly undercoating on it
.
I also have one set that is "chrome" plastic...just the outside tip/edge of the ribs are left chrome, the rest is painted. (all original factory finishes)

-"chrome" has a dulled look to it on sections and hope to clean it and bring it back to near original condition, BUT don't want to do more harm than good using the wrong cleaning solution and tools.

What do the Pro's/car show enthusiast guys use?