making a background....sorta

thats the same type i used on most of my background (see pics in the link in my signature), so i don't think it is a styrofoam issue, i think it is a cement issue what brand did you use? and where in the world do you live? what country?
 
the type of styrofoam isn't the problem.. you should be using CEMENT.. i would imagine that CONCRETE would be too dry and heavy to stick to the styrofoam.
 
i am using quikrete
 
is it dripping off? or is it falling off?

if it's falling off, add more water.
if it's dripping off, add more quikcrete.
 
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