Help Need to paint a fortress

in the last days, I've worked on a massive construction for 40k, which i intend to use in Apocalypse games, or even in standard games but with particular rules.

It's basically the walls of a fortress (if you have ever seen Boylan's Helsreach, you know what I'm talking about), built with various polystirene material (TV packaging, polystirene foams cut, polystirene food trays…).
2 section, each wall is 46" long.


The question is: how do i paint them?
I've had in mind heavy use of spray concrete grey, then brush darker grey for details and some dark green for the upper parts (parapets and similar).

However i've never painted polystirene, so i don't know how to proceed.

I've heard advices that direct use of spray is problematic at best and the result would be unsatisfying for a number of reasons.
So the suggestion to bypass the problem(s) is: create a soft paste with vinyl glue mixed with water and chalk / plaster, then place it all over the surfaces, to give:
more weight to the terrain
more resistance to polystirene
a paintable / sprayable surface


but i'm here to hear also YOUR advices…


here is the actual VIP:

 

FORTRESS WALL N. 1

 

FORTRESS WALL N. 2

 

Interesting..i wonder how much surface can be covered by one of those.

Alas the description says:
International Shipping : This item is not eligible for international shipping.

 

Yep, Italy.

 

Well, i've got both pva and sand.
How should i apply the mixture on the surface ?

 

The way I usually apply sand to surfaces is like this:
- Make a mixture of PVA and water, 2:1 approximately.
- Use an old, big brush to put it onto the terrain.
- Just throw the sand on there. It should stick
- wait until it is completely dry.
- Paint with wall color from the DIY store. grey.
- shade with thin mixture of black wall paint and water.
- carefully drybrush with a brighter grey (mix grey and white).

EDIT:
Try this on a small part first, obviously. I use a finer styrofoam material so it might work differently on yours.

 

Vallejo is a Spanish company, so it's just as easy to get their products in Europe through other websites.

Though, for your intended purpose, I think using a pre-made solution for a larger area would quickly become expensive.

As others have recommended, make a mixture of PVA (or if you want to make really large areas and looking for a cheaper solution, carpet/floor glue as it comes in 20L containers and water.

I would strongly recommend painting this mixture onto the polystyrene for several layers to ensure it is all sealed up, before gluing sand to the surface.

 

Yes, it was my intention since the beginning: a solid backup for the defenders.

The knight needs a small platform to be raised to the needed level, but the result is threatening

 

VIP .
In the end i have decided to do a test on the first one with PVA, water and paper.
The next step could be the placing of some thick paint..