Green epoxy can interfere with the cure of some varnishes. Also imprinting will occur if the final sanding is done too early. (Your epoxy around the glass is well cured so no concern for you.) Also bright speckles in the cloth seem to show up in time where glass is exposed and not sealed with epoxy. So the skim coat is a good idea. I wait two weeks before varnishing. Since your hull is smooth before adding the skim coat, you only need to remove the dust bumps and provide some tooth for the varnish, 220 grit should be coarse enough to do that and avoid cutting through the skim coat. I have found that the foam brush strokes in the varnish are the same profile as the scratches from 220 grit sandpaper, so there is no need to go any finer.