I've had good luck with the Novus products.
Usually you can find the 'Heavy Scratch Remover', the 'Fine Scratch Remove' and the 'Polish'
Amazon knows all:
Amazon.com, query: "Novus Polish Kit"
They sell all 3 a kit for about $15 (slightly more or less from different vendors...)
I've used it on VTVM dials (Heathkit) with GREAT success. Also other clear plastic dial lens, and also for plastic radio bodies... Some of them, if not too many ding, look spanking new. I had a little 1955 Philco tabletop that worked out that way. (Usual story, been in an attic of a repair shop for about 40 years...etc). That think looked new.
But it depends on the type of plastic, for sure...