I don't think you need a filler coat on foam.   Make sure it is sanded nice and smooth and then vacumn the dust off real good if not better.   Put the SLC covering on.   Doesn't take as much eat as Monokote.   

the filler coat referred to is light weight spachtling compound(Red Devil, DAP, or house brands).  Thin it slightly with 1 tbsp. of water-based varnish to 1 cup of compound.  Brush it on, working it into and small holes in the foam.  If the foam is in spec EIFS foam it won't have many holes.  Sand almost all the filler off, it's mainly to level the surface if there are any dents, wire marks, etc. and get a smooth base for the film.

Applying the covering will raise some grain from the foam, so apply a good coat of primer applied as dry as possible.  Block sand it all off.  Applying silkspan does the same thing, but weighs more and takes more time.  I like rounded tips.  Applying silkspan strips and filler directly to the foam before covering allows you to fill in and smooth the foam when the tips are sanded.

Bill's reverse method works well too.  Takes a bit more time and weight.

Phil C