I'm not sure what you're saying. Did you already buy the stuff in the tub from the supermarket and you want it to look perfect? And you want to do a crumb coat? I'm not sure what you mean by crumb icing? Sorry, I'm just trying to understand so that maybe I can help answer your question.
Oh, I see. I'm not too sure. Maybe dipping your spatula in hot water to get it smooth. I haven't tried w/ store bought, though. Make sure you wipe your spatula off first and just do it dry. Have you heard of the Viva method? That may work, I think store bought crusts enough if you let it sit a couple of minutes. I'll get the tutorial link for you. HTH.
Here is the link. You can skip a couple of steps, like rounding the top edge and using the soda bottle tool. Just do the smoothing tecnique and that might work. Good luck!
http://www.cakecentral.com/article10-How-To-Create-Faux-Fondant-The-Paper-Towel-Method----Viva.html
Once you're done w/ the crumb coat, you can probably just start right in w/ the frosting. Do a little test spot just in case. If it starts pulling up the crumb coat, let it sit a couple more minutes, or pop it in the fridge.
Check out this link and you will be so happy...the secret to very smooth cakes... written by Melvira who is a funny and helpful CC'er...
http://www.cakecentral.com/cake-decorating-ftopict-42675-.html
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