Restoring lead.

Hey lads. I was out working with a guy a couple of days last week. He was doing a roof while I did the pressure Washing. Only thing was, when he was done, the lead in parts had reacted with the hypo and discoloured. I know there's a chemical to restore this, I just can't think what it is at the moment. Any ideas? 

He wasn't too bothered about it but I wouldn't be too happy leaving a job like that. Some people are far too fussy and would see that as a chance not to pay or just be difficult. Also, is there any easy way to avoid this issue? Would a quick spray of wd40 or something help? 

I think it's called "Patination Oil" they used to use it at the construction company I worked at.

If you damp down the lead whilst applying the hypo mix and keep it wet with water it will stop it happening in the first place but I accept this can be difficult on some jobs , if it’s accessible cut a lemon in half and rub it on it whilst it’s still wet thei will remove it but won’t work after , I also cannot remember the name of the stuff I have never used it but the videos I have seen of it seam to have mixed results , we just keep it wet and rinse off straight away and generally don’t have a problem. 

It wouldn't be oxiclear would it?