Steve, on my 18 foot restoration, one quart of Epifanes yacht enamel gave me two full coats with a little bit leftover.
On the interior, I used Totalboat wood filler/varnish primer for two coats, then Halcyon gloss varnish for two coats, and then Halcyon satin varnish for one coat. I used a quart for each of the first two, but only a small amount of the satin. I assume the early coats soaked in, then by the time I got to the satin it only needed a little to cover.
If you use Halcyon, ignore their instructions saying not to thin it, because you will get runs and won't be able to keep a wet edge. 5 or 10% water makes a huge difference. I hated Halcyon the first time I used it, but then I wised up and now love it.
