By grainy, do you mean gritty?  Kind of like there was sand in the icing?

Yes...exactly like that...

Yes high ratio shortening makes the difference.  Is it true that the FDA is going to ban trans fats?

I haven't seen anything definitive yet but it is being studied (banning of transfats).

Four things that may contribute to gritty icing.

Not to start a controversy here but I still only buy pure cane sugar - if it doesn't say that, I don' t buy it.  I still believe that beet sugar creates problems.

Also some cheaper brands of powdered sugar are not fine enough. Might help to use a sifter with really fine mesh to sift.

Yes it helps to use high ratio shortening as it does incorporate the sugar better.  But sometimes I still use Crisco.

Adding salt to your icing without dissolving it in liquid first.  Dissolve it in whatever liquid you add to your icing.

I believe you have already been given this answer, however I am new to baking for others and have been working on recipes and really getting my cakes to "bakery status". I have had this problem since I started, and as of this past Friday ill never have it again. I went to my local cake supply (30 miles away) and bought high ratio shortening, it worked GREAT!!! I use whatever powdered sugar my store has in stock, sifted it and mixed it with the shortening, added in favors, and its the best! crust on its on, not greasy, smooth, and overall better!

This is a VERY OLD post :)  But for anyone new looking at it I'll point out that *most* recipes say to mix only enough to incorporate ingredients.  If you mix at low speed for much longer you will have smooth, wonderful b'cream. 

Try the recipe "2 of everything icing".  (it's posted in the recipe section on this board) Yes, it takes some 40-60 minutes to mix up but you will have the smoothest, most wonderful tasting b'cream there is :)

lol sorry , thank you! im learning the site!

I find using a really good quality powder sugar makes a huge difference. I also add a little white chocolate ganche to ALL my buttercreams and whip it all together at the end. Comes out smooth. Makes a huge difference to the stability also. 

I only use powdered sugar and never had grainy icing.