It depends how deep and where the scratch is. A scratch on the title label is much worse than a scratch on the optical side. If all methods are tried and at least a couple of dvd players (including PC dvd-rom) still failed to play it then it's damaged beyond cleaning. If you really loved the movie and buying a new one is out of the option, then downloading it from torrent sites could be an alternative. You can choose the quality you like, save it in your hard drive or burn it to a disc. But then again doing it has legal issues.