We know flesh is stronger than steel in the One Piece universe but that wasn't really the point here. It wouldn't matter if the stuff he uses dissolved after every use because it would still hit before it breaks. He could just take another one. However, how is he going to get another pair of hands, eh? Like Assertn said, he was already wise enough to use his sandals once.

Believe me, not every detail in a plot, even a plot as good as in One Piece, can be explained sensibly. Sometimes an author simply ends up with odd solutions for the sake of the greater story. It's our duty then to point them out for our own amusement.