The Carnival Town of San Faldo was holding a festival where people wear masks, but more importantly, it's also a major source of steel for shipbuilding, which is probably why Franky went there. It would be hilarious if he rushed off to buy steel from a new Baku Baku Factory in Faldo.
I think it actually makes sense if he went to Faldo to get something that only a steel manufacturing town could provide him with. After all, he and the other Franky Family guys were saying that with the 200 million beri they could finally get what they've always wanted. I wouldn't be the least bit surprised if it was a ship related item, and if he's going to San Faldo, why not where a mask? It's not like the people of W7 can't recognize him with the mask on.