I'm building a fiberglass/epoxy cockpit coaming, molded around foam, similar to Vaclav Stejskal's Composite Coaming instructions. How many layers of what weight of fiberglass do I need to build up a good rim?

What matters is the final thickness. For mine I laminate enough carbon fiber and/or glass so the final thickness is just under 3mm (1/8"). A thinner lip is tough on skirts. Figure roughly at least six layers if you're using all 6 oz plain-weave cloth. I cut my strips on a bias from regular cloth. The edges are less prone to raveling and they conform well to the double curves.

BTW: I use the carbon purely for appearance. With a wet hand layup I doubt there is any weight savings or increase in strength.