Hi

Having made a rudimentary model of a ship for presentation purposes, I'd like to color the thing. Now, the hull is made of two lofts; one for the aft section and one for the bow section. These lofts comprise several surfaces, but there aren't any surfaces lining up with the waterline at nominal depth. From the keel up to the waterline, the hull must be painted black (or at least a darker shade of the one above), and I can't figure out how. Do i offset surfaces with distance of 0 and split it at the waterline, or is that unecessary?

If the description doesn't make sense, here's an easier analogue: How do you apply appearances to a square surface between the bottom-left, bottom-right and top-right corners, such that the appearance appears on half the surface?

Thank you!