Eaglecash mentioned Apoxie Clay, and I'll recommend another product from Aves, Apoxie Sculpt. It's also a 2-part epoxy. It's easy to work with, mixes easily, can be smoothed with a wet fingertip, cures rock-hard but sands well enough. I use it on figures metal, styrene, or resin.
Like Eaglecash, too, I used to use Miliput. I have no complaints with it, I just tried Apoxie Sculpt because other figure painters and sculptors I know use it. Apoxie Sculpt is a bit more finely grained than Miliput's regular formula, but Miliput does make a super-fine grade, too.
Here's Aves' website: https://avesstudio.com/
You order online, though it's also available in many art supply stores (like Richard Biick's).
Another product that would work is Sculpey, but it has a challenge, in that it's heat-cured. The instructions recommend a low heat setting, and many who use it, use an oven to cure it. That makes it impractical for styrene, especially. But a friend of mine uses it with styrene, resin, and metal, and he uses a hair dryer to cure it. So it can be done.
Hope that helps!
Best regards,
Brad