What I do is have a filler/glue mixture which I mix to bring it to a viscous state so it can be shaped and create a strong, thick but workable product. It is a lot harder than filler but sandable. After fairing I paint two layers of pure West System ofer the top, sand and paint. If the hulls are inverted in the sun you can often get three layers in one day.
I have had great, quick and tidy results with this. Just be sure to mask properly and there should be no probs.