I dont have any reference picture of the exhausts themselves, but i do believe the fumes and sea water would be very harsh on hot metal.
The exhaust main box will show gradient rusting process.
The first stage was to give the box required texture. That was achieved by dubing the MMD green putty using hard brush. Than i pressed some indents with pin.
Painted flat brown.
This is the first stage. Next will be salt treatment
Stay tuned.
TBC...