Westy, there are lots of trowling compounds. Interlux sells one called Surfacing putty that sounds a lot like what you used. I have used it, but it is expensive and sets up really fast so I switched. Also Interlux white seam compound is basically what you describe, but it takes longer to cure and only gets really hard after a year or so. These days I spot fair with an epoxy or vinylester product. I have used AwlFair, Petit Easyfair, 3M vinylester putty,and System 3 quick fair. The vinylester putty cures much more quickly than the epoxy based compounds. All adhere well and are easy to sand.