Because the car panels are galvanized on the inside and outside the chance that you have an inside out corrosion is very very low. On a not galvanized car this would be looking much worse.
To get this repaired the galvanization should also be renewed – which isn't an easy fix.
The warranty for an inside out corrosion was 10 years. So in 2016 you're no longer in this time frame with a 2005 car. Also it's not inside out.
Let the repair be done professionally. Avoid to much loose gravel and have fun with your Boxster again.
Regards, Markus