Fermcap does a great job of killing boilovers as well. I've found that adding a few drops right when boil starts will do the trick. I've tried using it when the hot break happens, but it's a crapshoot at that point. It's kind of a PITA to stand over the kettle waiting for the boil, but I'd rather do that than clean up nasty gunk from the outside of my keggle.