I did the back of my 180 in black pond foam . I found it very easy to do . The key thing is to be as random as possible with the material as you are applying it . Also allow the tip of the nozle to sit in a few places to build up the thickness randomly as well . I did the 30X60 background in about 30 minutes and used 4 cans of material
The only thing I have noticed is that some of my tangs have taken to chewing off some of the foam when they peruse around for algae to eat . I find the bits in the sump but it does not seem to be harming anything. In some places I have been able to take sps frags and literally stick them into the foam backdrop to grow