I use this stuff for molding fiberglass and it works great. The slow hardener gives a mixed shelf life of 72 hours, but it still cures in about hour after shooting it. It does say on the can that it is not sandable, but I have found that the black sands quite well anyway. It is not a primer surfacer so it does not fill defects very well, but it does sand much easier than paint and it covers and flows out very well. An added bonus is that since it is epoxy based the chemical resistance is outstanding.