: Do I sew on the D-rings for rigging before or after sealing the skin?

Well, there's usually not a lot of sewing going on with SOF rigging. It's often just running lines through holes drilled through sides of the gunwales. If you want the more modern deck rigging that one sees on commercial kayaks, then screwing loops of polyester or nylon webbing into the gunwales works well - with or without D-rings. I'd do this after coating the skin. Use a hot nail or a soldering iron, etc to melt the new edges of the fabric.

Here's a pic, from the CLC website.