concrete comes right off

[yilduz]

So, I tried doing that thing where you use Great Stuff expanding foam for the background on a tank, and then you "paint" it with cement. After it dries, though, it comes right off when I rub it with my finger. This is even after a few coats. I'm not sure what I'm doing wrong. Is this common?

edit: The title is meant to say "cement," not concrete.

 

[Mogurako]

First coat needs to be really thick and the surface can't be to smooth or it won't stick.