I sanded a tuperware fajen and not the Stevens 200, but I'm sure it is much the same.
Just clamp the stock (barreled action removed) in a padded vice with the butt sticking out, cut a long, thin strip of 150 grit emory cloth or sandpaper and, holding one end in one hand and one end in the other, do the "shoe polish" back and forth over the mold seams. Flip it upside-down to do the underside, and flip it around in the vice to do the fore-end and underside.
Use a sanding block to get the edges along the sides.
If you're going to paint the thing yourself I'd highly recommend sanding the whole thing lightly once you've got the seams level just to give the paint something to which it can adhere, then use a lintless towel soaked in acetone to clean the whole thing off before spraying on primer. Remember to use gloves when cleaning it off to keep it perfectly clean so the paint/primer adheres best to the plastic.
I personally don't care at all for camo pattern stocks, binoculars, turkey shotguns, range finders, etc, but found that I could put together an effectively ellusive paint job without going "camo" in the strictest sense:
I kinda thought that Gotham Gray Stonetouch made for a pretty decent snow camo...