Typically overprint or "double-strike" is used for either printing on clear or translucent. It adds a second (or third/fourth) identical layer of ink on your pass to give you optimum density of clear or semi-clear material. The problem is finding the right density without bleeding. I've never had to use more than 2x overprint on either clear or translucent. Much more than that and your color goes to hell, you waste a ton of ink, and your still have a crappy print!