I just started the varnishing process on my Guillemot, using System Three's Spar Urethane Varnish over their epoxy. My question is this: Is it really necessary to sand between coats? I've seen previous posts that argue in favor of both sanding and not sanding. What are the benefits and pitfalls of each?

Additionally, I'd like to get a "Satin" look on the final coat- are there any tricks to this?

Thanks!
Todd