juust use the cheap natural grey.. but youll need sealer to.. mortar is basicly that stuff used between bricks on a wall... i dont think it would be smooth, it would have to do with the way you put it on..
heres a great link that will let you know everything you need to..
http://708designs.netfirms.com/custo...customcage.htm
and heres a link to my work in progress
http://www.ssnakess.com/forums/showt...threadid=63416
i have a few more platforms up now and my floresent light in.
P.S. if your useing foam like i did use the BULLDOG GRIP foam board adheasive. i found it at home depo by the foam and in the caulking section... it has really good streangth.. i put 10 pounds on the end of that top shelf (the one in the middle, the 10 pounds was about 10" from the back of the box) it held without a problem