Before I assemble a figure, I prepare the body:
Next step is to check if all joints function properly: range (also left vs. right side) and stiffness (i.e. holding its pose).
Original Cool Girl bodies usually need very little preparation but most other bodies do.
Uneven seams between joined body parts can best be leveled by shaving the seam with a used loose hobby knife blade. I prefer using an old X-Acto Nr.10 blade as this has no sharp point that could dig in and make scratches.
Take care not to create any flat spots.
Misaligned edges not only spoil the looks but can also cause clothing to snag and get damaged, so I prefer to trim any misaligned or sharp edges using either a hobby knife or some fine sandpaper.
With some bodies the looks can be improved by trimming parts, this depends on the type of body:
After all parts are trimmed, shaved and sanded, I polish the plastic using a microfine sanding sponge.
In case of a figure with a high gloss finish I polish the entire figure.
Finally I buff the plastic using some coarse wrapping paper or a small felt wheel on a motor tool running at a slow speed until the plastic has an even satin sheen.