I made ancrusting buttercream and thought it was perfect until I went to smooth it.  It kept getting the little holes in it and I couldn't get it 100% smooth.  I followed the directions to a tee and made sure to mix on low only so I didn't incorporate too much air.  It was a half butter half hi ratio shortening recipe.  What else could be the problem?  It wasn't a big deal this time because I was just practicing the smoothing and ended up making it messy to look like water but I'd like to be able to do it when needed.  Any suggestions??

thanks!

I would put it back in the mixer with the paddle attachment and mix on the lowest setting for a couple of minutes. If I let my buttercream or royal sit too long, air bubbles appear as well and that is what I do to remedy it. 

You can also just take a spatula and beat it by hand after you have made it, this will knock a lot of those pesky air bubbles out of it.

Thanks!!!  It was sitting for a few hours so that's probably it! 

Yes, b'cream will bubble when left at room temp for more than a few minutes.  Letting it mix on the lowest setting your mixer has for up to 5 (even 10) minutes should cure your woes :)