what is the proper amount of icing used to crumb coat a cake before covering with fondant? Any tips on getting the icing super smooth underneath? thanks

What are you using for icing? If standard crusting buttercream then they suggest that you use just enough to cover the color of the cake under the icing. How to get crusting buttercream smooth? Put the iced cake in the freezer for 15 minutes then take a pot of boiling water and place it next to the cake. Take a metal offset spatula and dip it into the boiling water, shake off once over a sink then start smoothing the icing. You'll need to dip the spatula blade frequently during this process.

The other way to get a smooth icing if it is a crusting buttercream is to use the Melvira method for smoothing, That in combination with viva paper towel and a fondant smoother (or computer paper if you don't have Viva paper towel) will give you a smooth finish.

http://cakecentral.com/tutorial/quick-easy-smooth-icing-using-a-roller-melvira-method

I used to use the hot spatula method and the computer paper until I found this and was able to get Viva paper towels.

After last weekend I have decided I'm never crumb coating my cakes again. I fill my cakes and let them sit for several hours or over night before decorating to let some of the air release from the cake so I don't get air bubbles. I usually don't do a crumb coat and never have any problems with bubbles in my frosting but I decided to try a crumb coat and kept getting air bubbles in my cake. I will stick with my large icing bag and my Wilton icing tip it has never let me down.

Ive been using a high humidity crusting buttercream. I smooth it out with hot water and the viva method and it's still lumpy underneath I thought I wasn't using enough icing so I used a little more next time around. Must've been too much cuz then I got ripples
I also had issues with icing blowouts. Most of my cakes were iced in buttercream and it seems like it's only a problem in the warmer months. I tried everything.. cake spackle, weighing the cakes down after filling them, changing the icing recipe and the list goes on. Finally someone suggested I only ice the cakes at room temp which includes the crumb coat. Once I did I haven't had problems with cake farts since!! I've been doing more and more fondant covered cakes recently, and was extra worried about bubbles under my fondant! No problems so far, fingers crossed. oh the trials of a caker!!