David,

My experience with West Epoxy (in Melbourne over a period of a couple of years) while I built a 7.8m stripper trailable yacht, was that it will stay green for about a day. But with the carbide scraper, it won't matter if it is green or fully cured. The scraper will take the tops off the runs and lumps a lot more controllably than sanding and allow you (with a little care) to get down to the mean level of the epoxy without producing hollows. Sanding tends to not only take the tops off the bumps and runs, but also almost unavoidably produce hollows around them, which then need to be filled again. (if you want a fair surface)