If it's not too late, I recommend using the Mr. Surfacer 500. It's thicker and, I find, does a better job on creating texture. Once you have applied either version of Mr. Surfacer, keep stippling it as it dries until it forms stiff peaks, like merangue. Once it dries the texture will be rather heavy, but then the light wet sanding with 600 grit will take off the tops of the texture bumps (unless you want a really rough casting) and allow you to generate whatever level of texture you want.
As was said, practice first. You'll find there is a fine point in the process where you get the sort of stringy peaks you want. After that you can actually start to pull the material off the substrate, which tells you you've gone a bit too far and it's too dry. If that happens, just reapply and try again.
Do not glue anyting to the cast surface as the bond of the Mr. Surfacer isn't great and the object is very likely to come off wihta small knock. Usually at the worst time.
I like to use the otherwsie crappy Testors plastic brushes for this as they are relatively stiff, stipple well and hold up to the abuse pretty well. Clean them with laquer thinner after each job and they last quite a while. Don't use an overly soft brush or one that's worth any money as you'll destroy it pretty quickly.
HTH
Paul
I thought using a brush (rather bigger than smaller) to apply it and than sand it down with a sand paper (I have 600, 1000, 2000) in order to create a rough (but not too much) texture. Any advice would be nice 
