Not easy! Depending on what you are trying to assemble, taking out the hands from assembly can be VERY difficult!!
If you delete the hands, there will be missing information from your product. Consider this: blue hands threading a bolt through a nut. Where one hand covers the held nut, the key will read as a matte, leaving the shape of the fingers transparent.
Unless you have shot parts to replace it, you will have strange little "bluescreen hand" shapes tearing the edges of your construction.
Better, although more difficult, production, is to build a rig that is attached to your assembly pieces from behind the subject. You can either drape them in greenscreen, or rotoscope them manually. Navigate the animation with the rig, or complete it in steps.