I use mostly butter in my buttercream but sometimes fight air bubbles which prevents me from having a smooth cake. How do I prevent this? What am I doing to cause this? What makes a buttercream a crust?

Make a full batch that fills the mixing bowl to completely cover the top of the blade so you aren't incorporating air.

You will get air buttles if the beater isn't covered with icing &/or you use too hight a speed OR don't mix it long enough. Mixing a full batch on low speed for 10 minutes should help eliminate the problem.
Try this recipe: http://cakecentral.com/recipe/2-icing

Take a look at this recipe : http://cakecentral.com/recipe/easy-buttercream-3