I'm planning on filling out ad smoothing some grooves on some 1/6 shoes and then painting over it. What's the best kind of putty or whatever to do that?
You could use 2 part epoxies like Magic Sculpt and Apoxie Sculpt. Both start to harden in 6 hours and you can use a little bit of water to smooth it out.
Apoxie Sculpt is good stuff. Finer grain than Milliput and dries like rock (and then it's sandable) You can smooth it with water while it's pliable. You can also add water to make a slurry and fill fine cracks.