Filling a CraftID is typically done to highlight the contents and to increase durability. Some CraftIDs lend themselves to being filled (wood for example), while require no fill (plastic for example).

A raw CraftID should generally not be sanded because the very small engraved parts may break under even a little abrasion.  The solution is to fill the engraving with something that allows sanding as needed.  My favorite is using colored five minute epoxy.  I color the epoxy with fine sawdust or a variety of products such as powdered pigments.  A filled CraftID can be embedded slightly proud of the surface and then sanded flush.