Like Rob says, follow the directions. At least the first time. As you get more familiar with a product you might be able to take more liberties. Regular old fashioned spar varnish will bond to previous coats without sanding, but that may not be the case with the Sys. 3 product. It may require a scratched surface for a good bond between coats. Some sanding between coats will often produce a nicer looking finish.

The gloss helps reflect UV and is useful as it is. If you want a satin finish anyway, rub it down with fine steel wool after the finish is good and hard. For a more matte finish use coarser wool. This process will generally look better than if you had used satin varnish to begin with.

: 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?