The buttercream I made for my daughter's birthday cake seemed to be fine when I made it and refrigerated it. The day to decorate, I whipped the BC prior to icing the cake then started to apply. It went on smooth for the first layer then all went bad...it started looking "gritty" and not spreading smoothly at all...What did I do wrong??

Did you use pure cane confectioners' sugar? If it doesn't say "pure cane" on the packaging (i.e. many store brands) it most likely is beet sugar, which can result in a gritty icing.

Did you put plastic wrap on the top of the buttercream when you stored it? If not, it could be that the top of it crusted, and on rebeating it the crusty bits got distributed throughout the buttercream...

I'm thinking that you meant by gritty it had holes in it, air pockets and not a gritty taste or texture.

I get the same thing when I let my BC sit for a day. My theory is that the beater does not get fully covered and air gets incorporated into it. It doesn't show up til it sits. If I use it fresh it is not a problem.

I use SMBC exclusively. I am going to try and up my recipe til I get the amount that covers the beaters and then I'll know if my theory is right. In the mean time I stir and rub it against the side of the bowl to get the bubbles out. At least o. My last batch. I haven't made a cake this week yet.

BTW I don"'t get this problem when making cream cheese frosting, no matter how small of a batch I make. So maybe my theory is wrong. But then again I don't mix my CC frosting nearly as long as I beat my SMBC frosting.

what is smbc?

Swiss meringue butter cream

did you sift the sugar before you put in in the mixer

If you keep spreading the icing over and over in the same place or go back over another one, sometimes the buttercream will have already started crusting and that will give you a gritty look to the icing. Keep your knife clean for those touch-ups.

I agree that it may have started to crust while refrigerated.

(I never refrigerate my BC.)