The potential damage is more to the paint finish than to the roofing itself, causing a risk of leaks. However, if the finish is damaged badly enough, that could potentially expose the metal sooner rather than later, leading to problems. You could paint over the roof in a process of cleaning it and then applying a high quality exterior paint, and perhaps create some sort of faux finish technique to get the look you want. The new finish will void any warranty that might be remaining on the original finish and it also son't be as durable as the original finish but it may get you the look you desire.
Or copy this URL:
https://www.metalroofing.com/spirit/comment/15794/find/
There is a post earlier in this thread asking what to do about rusted areas pre painting.
If you can find Ospho ( it is online) it is a dilute Phosphoric acid which if painted on to the rusted area turns the ferrous oxide (rust) into ferrous sulfate.
Wash the affected area with Ospho, let stand for a while, but do not let it dry, then hose it off with lots of water, the rust has been converted and paint will stick.
First started using it on my steel sailboat in the 70's with very good results. It is very easy to use and long lasting. It is chemically pretty much the same as Naval Jelly but is a liquid, seems to be a little stronger than vinegar but generally pretty mild.
I know this is a response to a very old question here, but thought I would comment in case there is anyone else with the same problem.
Or copy this URL:
https://www.metalroofing.com/spirit/comment/15795/find/
Thank for the detailed advice Tony. Would it be advisable to neutralize with baking soda?
Eric Novotny An informed customer is the Best Customer!
9/10/2017
Share
Or copy this URL:
https://www.metalroofing.com/spirit/comment/15796/find/
Have used Ospho for personal projects and in industry, never found a need for anything but water to wash off and dilute the acid.
If you had important plants under the runoff you might think about neutralizing the acid. It does not seem to affect paint or other finishes, have used it to clean up the chrome on bikes and to wash every piece of metal coming into a fabricating shop I ran years ago.
Note it is important to wash off the Ospho before it dries, as it will leave a hard scale. Depending on local conditions you may need to rinse within a few minutes or perhaps after an hour.
Isaiah Industries, Inc.
7/24/2017
9/9/2017
An informed customer is the Best Customer!
9/10/2017
9/10/2017
11/13/2017