How best to cover rigid foamboard & expantable foam on an incline

I have a long area that consists of 1/2" rigid foamboard smooth & 'shaved' with expandable foam (also shaved down) on an incline that I'm not sure how to treat.

Go over it with hydrocal or brush full strength PVA glue and sprinkle with medium Talus.

See the photos. (The 2nd pic, note the much darker 'gray' when I painted the expandable foam)

 

I would suggest applying a layer of "Lightweight Spackling Compound" to smooth out the area and add any contour you want. It is extremely easy to work with and very inexpensive. After that dries, I would then apply a coat of earth colored paint, then the PVA brushed on, with a layer of talus sprinkled on. Once that is set, apply another layer of talus and shape/arrange it until you get the look you like. Then soak it all with a 50/50 mixture of matte medium and let set. After that, apply some weeds and you will be good to go. Take your time and do it step by step. You won't be disappointed.

I have done this exact process in a few areas on my N-scale layout. Vary the amount of vegetation based upon "how long" it has been since the rock fill was placed to achieve the effect you want

 

Here is that same portion as that 2nd attachment with gray talus applied.

 

Separate area of the same layout. Different problem.
Long story short, I have a high inclined surface with Hydrocal over Sculptamold that hasn't been painted yet. I temporary placed small rock castings to dress up the surface which was minor detailed. My question is do I glue these castings to a unpainted surface as it stands, or remove then paint the surface, then adhere them to the painted surface?

The adhesive is liquid nails if that matters.

 

First thing with foamboard, peel off the plastic sheets from both sides. They're hard to see, but trust me, they're there. This allows whatever you put on the foamboard to adhere. (Foamboard is porous without the plastic sheets.) Otherwise, whatever you put (glue) on will pull off easily, plaster, sheetrock mud, cork roadbed, etc. Then lay down a single layer of paper towel sheets dipped in thinned sheetrock mud or plaster to which everything will adhere. This gives you a firm base to build your layout.

 

If you glue to a painted surface the paint could easily be peeled off with the glue. Especially water based.

 

That is not foamboard, it's Hydrocal! Re-read my previous post.