Hello,  I was wondering if anyone has had any experience using RKT without the fondant, and using only buttercream to cover? The only way I can think to make a smooth design is with layers upon layers of buttercream but wanted to see if anyone has any tips! Thanks!

I have had no experience w/RKT But Have read that people sandpaper the item smooth   Use a zester file  

depending on what you want to do — you can whirl the Rice Krispies in a food processor then make the rkts and that will come out smooth and much more secure  — uses more of the cereal too — but nice and smooth 

I used candy clay to cover rkt before —

don’t know why you can’t ice them smooth though — with buttercream or royal  

I don't think you would have to use layers upon layers of buttercream.  I would treat the RKT like a cake.  You would want to do a crumb coat which essentially will fill in the cracks and crevices of the treat (I also think you could use ganache or candy melts as well...this would also probably help to really seal in the stability of your RKT piece).  Let that chill or crust over and then do another, thicker, layer of buttercream.  To help get a smooth finish on weird curves or tight areas get yourself a piece of flexible plastic/acetate.  

I have only ever covered RKT with fondant, but I still give the treat two coats of buttercream so that when I am ready to cover it in fondant it is pretty darn smooth.

I most frequently cover RKT's with modeling chocolate (candy clay), but I have covered it with buttercream and ganache.  Just like a carved cake.