Clean Up your synthetic stock with sandpaper.

There is a thread on the forum about cleaning up plastic/synthetic stocks.
Much advice was given: dishwashing liquid, dynamo laundry detergent, gun oil, and even a blow torch (!).

The trouble with my Remington Mountain Rifle stock was that it didn't just have ingrained mud, but was scratched up from 20+ years of bush bashing.
There's no way soap of any description would be able to clean it up!

So forget the Dynamo.... I needed the Heavy Duty Cleaner.

Enter the Orbital Sander with 320 grit sandpaper.

With all the scratches sanded out it was a 'matte black' colour rather than the original gloss black.
One can leave it like that, add Armour All to give it the gloss look, or spray paint it gloss black again.

Before:
Attachment 204353

Attachment 204354

Halfway through sanding:
Attachment 204355

Nearly finished:
Attachment 204357

Attachment 204358

I used a block and fine sand paper after a pig chased me off a cliff and I landed on my rifle on stones and it came out ok

one direction only sanding and be real careful of the edges id say you saved a bit of time with the sander but it will look even nicer if its all even

you can stick tape over the checkering if you want to relax a bit when you are near it

Don't knock the blowtorch! Melting the surface with a light application of flame can heal those fine scratches you have made with the sandpaper, although it does depend on the plastic. Don't try it on a fibre-glass stock though. And as originally posted, test on an inconspicuous part of the stock - perhaps the barrel channel.

With dirtbike plastics, applying a bit of heat with a heat gun is a great way to bring them back to life. There are plenty of videos on YouTube covering this. Just have to be careful not to get to close and melt the plastic. And as @Bol Tackshin mentioned, test on an inconspicuous part of the stock first.