I haven't tried doing that icing, but maybe it has the same problem as wilton's icing? I use the class Bc icing and in the beginning I had problems with it being grainy too, but the solution is to sift the powdered sugar. Do you do that? And if you do sift, are you mixing the ingredients in order, as in mixing the dry ingredients last? Because if you put the ps first and then liquids, it can form clumps. I heard it can also happen if you let the icing stay out for a while before storing it or using it. Maybe it's the same for SS's Bc? Hopefully someone with more experience can help you out 
did you use sweetex or crisco? i tried it with crisco just to see what would happen and it was really grainy
I actually have this problem when I make my buttercream all the time. I use storebrand shortening and whenever I overmix it, it feels like theres sand on the icing once I taste it. So what I do is that I add more milk or any liquid mixture, then I keep beating it... It turns out fine. Although it feels too fluffy but still workable.
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