Essentially after bondo the peices are casted into smoothcast or something more durable and wearable than the glass, and also light weight, i'm on the rpf too though I never cast my peps cuz that stuff is expensive. on the edges and insides I sand down with a disc on the drill and coat with some felt or thin foam so there's no digging or scratching on the return edges.
For scaling generally the chest, cod, back and butt are the same width, lay down or put paper on a wall, mark points on the sides of your rib cage at the widest point which is around your underarms, then your waist. measure the distance and add an inch or 2, then convert to mm and put as the width in change scale.