So I'm trying to prepare wayyy ahead of time for my sons birthday in January. I want to make a Lego Batman out of rkts.. I've never used rkts before, I've done a little research and I'm wondering which is better buttercream underneath or candy melts? If candy melts are better do you melt it and just brush it on??? I would hate to have a lumpy batman on my hands. Please help I'm the only one I know that knows anything about cakes lol 

I always use candy melts.  They dry harder than buttercream and the topper will be easier to handle.  I usually put a good layer of candy melts and then after it dries I can smooth  it with warm hands or hot spatula.  I've also seen people "sand" down their rkt topper with a hand grater before they cover.  You could try that as well.  Hope that helps you a bit :)