I'm likely to be building a large sink this summer. But it occurs to me, a darkroom sink isn't like a boat or a whisky barrel or a bath tub. It doesn't have to hold water for a significant amount of time, and it doesn't have very much depth of water in it - it's probably never plugged or stopped. It's really more to catch spills and give you a working surface with high sides to catch splashes and make rinsing it all out (and cleaning trays and so on) easier on your floor and clothes. I'm assuming quality plywood; hold it together with polyurethane glue and screws; any exposed plywood edges get oak or hardwood strips solidly glued to them, with no gaps and really good crack sealing. Extra credit, design it in a way that water flows to the drain, and possibly throw in something that keeps trays level while maintaining a slope to the drain.
I expect that properly coating it with porch paint and whatever sort of primer is best for porch paint on wood, and you'll have a sink that will serve you for years. I know many people use 2-part epoxy coatings as well, some use fiberglass, some poly, some use spar or marine coatings. Maybe you want a paint or coating that can expand/contract with the wood. But it doesn't seem remarkably worrisome, though there could be aspects I haven't considered. It seems like some people over-engineer them since the word "sink" puts us in a "kitchen sink" mindset, a vessel that may have to hold water in quantity for some time.
