Another reason line makers are so secretive is because line weights aren't as standard as you might think. Industry standards require that each line be weighed only in the first thirty feet of line. After that they're free to taper their lines anyway they want. Research & development are big things in the fly line business & everybody's looking for the next big breakthrough that will improve their lines even more. So if you start asking a line company questions about specific ingredients or processes that go into making their lines, you'll be met with kind, but firm, 'none-of-your-beeswax' answers. Can you blame them?
All that aside, this video shares a lot of how they make fly lines - at Rio. I think this is a pretty interesting look behind their secret doors. I think you'll find it interesting too.