I know Shirely and Antonia use this, do you ladies or anyone else have advice on smoothing it out? I just did a basic crumb coat so far but the top is a bit thicker and looks like I may have rewhipped it too much, have some airholes. Do any of you use the icer tip when applying? Even sides are one of my biggest problems. If I can figure out how to smooth this so nicely, I'm sold on it, the taste is definitely worth it! TIA 
I don't use the icer tip, just an icing spatula to apply and a chocolate scraper or wallpaper scraper to smooth. I have never had airholes in that icing, I don't kno what could cause it. Did you switch from the whisk attachment to the paddle when you were adding the butter.
Another method that makes a nice looking cake is Jeff Arnetts upside down icing technique. The only thing I have found is you have to have a really smooth paper of some kind or you end up with some lines or wrinkles in the top. It makes beaiutiful edges though.
Not too sure about the airholes but maybe you whipped too much??
I saw in Toba Garrett's book she applied it with a simple star tip all around the cake; no need for an "icer" tip.
As far as smoothing it, have you tried the hot spatula technique?
I smoothed it as much as I could and then stuck it in the fridge a bit and smoothed some more. It did get really soft at room temp though and not just in bag from hand. I had trouble with damming with it for mousse filling. Had problems so I tried to add xxx sugar to it for damming to help but it made it worse... like it broke it down. Not sure how folks do detail with it. It was wonderfully delicous though... I used Shirleys recipe. It also seemed to give my fondant a bit of a problem... slippery, etc. I will try again to see if something Idid wrong...
I agree with Shirley. I use the plastering tool for applying it in one smooth stroke. Throw out those plastic pastry scrapers and go to a hardware store for the BEST tool under $2. They are solid, way larger, come with handles and many different sizes. Nothing I have ever used is better!
My best tips are to torte, fill & the chill the cake in the fridge or freezer for 10 minutes or so. Then remove it, apply the crumb coat and put it back to chill. Take it out and apply about twice as much buttercream as you'll really need. The smoothing comes in TAKING IT OFF the cake....not in trying to build it on. Don't play with it too much, you'll only cause more marks and imperfections rather than smoothing them out.
I like using freshly-made IMBC rather than thawed and re-whipped. It should be about the softness of whipped cream when you use it. I whisk in the butter and then switch to the paddle to give it a good beat on the highest speed for 2 minutes or so. The air bubbles are gone and it's the perfect consistency all in one.
I definitely don't recommend using a warm/hot knife in trying to smooth. You'll find that heat changes the colour of the buttercream to a bit of a yellower shade because the warmth actually melts the butter ever so slightly....causing a patchy look to your cake. 
I use a recipe by Colette Peters.....actually I usually make a half and half mix of this and regular BC (half butter/half crisco) ....gives me the best of both. I get most of the taste of the IMBC and most of the workability of the BC. I also use the hot water on the spatula method of smoothing. I use a bench scraper around the sides and a larger sheetrock spatula for the top.....works pretty well. I always ice the sides first just by scooping some frosting up on my spatula....I tend to get it too thick if I pipe it on. As for the air bubbles, yes I get them also, but they are easy to fix. After mixing my IMBC (I use the whisk attachment for the whole thing) I just kinda fold the mix with my rubber spatula.....helps to smooth it all out. After your crumb coat, put it in the fridge, after a couple of hours it will firm up and make putting on your final coat easier.....the cold will help the top coat firm up and smooth out better. If you have overnite, I found that you can put parchment on the top and the next day it will come off pretty easy and give you a smooth appearance ( took a hint from Jeff Arnett's tutorial). Good luck!
(hope there aren't too many mistakes in this....cannot type when I'm exhausted! )
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