Hi Vick,
I've fixed -- or tried to fix -- more than just cleaning marks. I picked up a severely damaged thin Tele-Elmarit (Leitz 90/2.8) for a few dollars at a thrift shop. The rear elements were surprisingly good, but the front element had a strange sort of decay on it, as if a solvent had worked its way into the glass, or at least into the coating. The lens was unusable as it was.
Not knowing what it was, I tried removing the substance at first with water, then with alcohol, then acetone, all to no avail. As I had nothing to lose, I finally set out to repolish the element. I'd never done that before, so my first efforts, with the polishing compound I'd gotten with a Dremel tool kit, probably caused some damage. I looked into telescope-makers' methods a little, though, and eventually got a set of grits and polish on the 'Bay. I don't have the seller's name handy, unfortunately, but if you search for cerium oxide I think you'll find some sources. Cerium oxide is a fine-grained polish -- much finer than grit -- that is used on optical surfaces.
I polished by hand until I felt I'd never finish, then gave in and used the Dremel tool with a buffer as gently as I could. Eventually I reduced the damage to a level at which the lens was usable, but there does seem to be some irregularity in an image reflected from the surface of the front element, indicating that I've changed the refraction slightly. The lens does take much more acceptable photographs than it did, but I'm not so sure it's all that a Tele-Elmarit could be. (I'm sorry I don't have any test shot scans handy, but the test shots taken wide open were still rather hazy, while the lens seemed reasonably sharp and contrasty by F5.6 or F8.)
Eventually I bought another of the same model lens here on RFF, one with the more common haziness in the rear elements and a much better front element. When I have a little free repair time I'm going to make one lens out of the two with the best elements -- and test it against a lens comprised of the worst elements just to see how much difference I can see.
Before buying another lens I looked into just getting the front element repolished professionally. John at Focal Point felt it wouldn't be worth the cost that would be involved, as the setup for one lens element is as elaborate as for a batch. But he was able to tell me what, in all likelihood, had caused the damage. Apparently a rubber lens hood left in contact with the lens surface for a long time can decay and damage the coating in just such a manner!
I've strayed from your question quite a bit, though. All in all, I wouldn't advise undertaking a repolishing if the lens is usable as is, and even if it isn't I'm not sure it's worth the effort. I've never made a telescope lens, so I don't know just how one manages to give the surface a proper and uniform curvature. Clearly I failed at that with my amateurish methods, but perhaps someone with more patience and skill might manage it. As for coatings, they are applied in a vacuum chamber, and multicoating requires several coatings of different thicknesses to cancel reflections of different frequency ranges. I can't imagine how that could be done without an expensive setup and quite a bit of experience, not to mention the proper compounds.
Unfortunately, it seems that professional repair of an element is quite expensive. I've thought about seeing if I could measure the focal length of damaged elements precisely and buy a multi-coated replacement element from someplace such as Anchor Optics or Edmund Scientific. But even their large selections of sizes and focal lengths are probably unlikely to include the exact element one is seeking.
Good luck in any case. I wonder if others have other suggestions... Are there compounds with a refractive index similar to optical glass that will fill scratches?
