Hi David, this is an interesting project. Is it an art work that is going to be hung?
I think your biggest job is going to be smoothing all of the contours without loosing any definition. Most of the surface you have exposed is end grain, which is hard to sand and will be very absorbent. I would hand sand and if the piece is going to reside indoors I would be inclined to use shellac, brushed on. If it is going to be outdoors well you could seal it with the same stuff you put on your lovely surf boards.