I'm certainly no expert, but that looks like paint to me.
BK
Edit, I found a CAF facebook post that says they strip and repaint her. They explained why they paint her and not polish her. So, that's that, she's painted.
"Folks always ask why FIFI doesn't go with polished aluminum. At the operational tempo of the Commemorative Air Force B29 B24 Squadron, maintaining that polished finish would be completely impractical, not to mention the amount of sheet metal work it would take to pull off. Paint also protects the skin for the next 25 years!"