Couched cord or filoselle is useful in covering the raw edge of the onlay, not so much masking the joints as making them sightly.
It disturbs the breadth, which is so valuable a characteristic of onlay.
onlay on·lay (ŏn'lā')n.
A cast restoration, usually made of gold, that is attached to the occlusal surface of a tooth.
A bone graft applied to the exterior surface of the bone.