Did you guys make a concave structure that remained with the sink, or a proper plug mould? In other words, did you:
- build a sink out of wood and line it with fibreglass, leaving the inside/working surface as raw GRP then install the whole thing, or
- build an anti-sink out of wood, line it with fibreglass, pop the wooden plug out and then just install the fibreglass shell?

I've seen the former done a couple of times where the mould is the frame, and it works well but is kind of ugly. The latter appeals to me because if I make the plug smooth enough, I can get a nice smooth finish on my sink and I can give the mould away for someone else to use - I know a guy who does fibreglass for his living that will supply me the raw materials and he could maybe use it if it was any good. The latter case is a little more difficult though because you have to mould enough structure into the fibreglass for it to be self-supporting and carry a large uneven load without cracking; in the former case, the wooden mould is always there as support.

Dan: what is this "marine paint" of which you speak? There are of course many, so if you know anything about the particular one you used that'd be very helpful to me.