I recently acquired a very inexpensive Zenobia with a 4-element Hesper lens. It needed cleaning on the back pair of elements (quite a bit of fungus) and I got that cleaned up nicely but the front element focus helix was frozen solid. In trying to free up that element, I gripped the brass barrel too hard and cracked the second element.
There was a lovely crescent shaped defect that took up around 1/4th of the element and totally
ruined the lens.
...or did it? After moping for a day or so, I decided that I had nothing to lose and put some UV cement at the edge of the element and let it sit. Lo and behold, after a few hours, the UV cement had completely "bled" into the crack and made it almost completely disappear. I put it under the ultraviolet light for a while to cure and now I actually have a pretty nicely working lens. Here's a sample shot:
Even wide open, my test shots did not show anything horrible - no flare and no discernible loss of contrast. Definitely not a "proper" repair, but certainly a way to salvage a pretty nice lens.
Hope this may help someone else...