In reply to JohnV:
If it is quite big then you are probably best having it patched professionally.
For small gouges 'down to the base' you can get 'metal grip' which is supposed to bond equally well to p-tex and the metal base. For small gouges down to the base I have found Araldite Rapid quite good!
For good technical advice you might like to consult www.thepisteoffice.com