I also built that model years ago when Tamiya was offering a special version of the kit.
If you're looking for an interesting reference book, this one is really good: I-400: Japan's Secret Aircraft-Carrying Strike Submarine: Objective Panama Canal
If I recall correctly, the hull was covered with a rubberized, black coating to deaden sound waves. I didn't model mine with this coating though. I used the two tone grey and red scheme.
- Jesse