The rock faces on Peter MacFarlane's N-gauge layout are some of the best we've ever seen and what's equally amazing is the simplicity of the technique he's used to make them:
Pete uses "Kingspan" foam insulation sheets which are normally used for cavity lining. Note that this is not the usual polystyrene foam that we are used to seeing on TerraGenesis. This stuff is pale yellow colour and covered on both sides in an aluminium foil. With some other makes the foil is also paper backed, but this is not important for our purposes as it will be stripped off anyway.
The foam comes in various thicknesses from 20mm to 170mm and the best one to use will be largely dictated by how much relief you want on your rock face. This in turn will depend on the scale that you are working to and the type of terrain required. For instance a Quarry will not be very deeply indented, nor will a sheer cliff face, but a mountain side may well have deep indentations and you may also wish to have pathways cut into its face. I usually model in 1:160 scale and I find the 2" or 90mm thickness is ideal.
Another factor is whether you will turn your raw sheet into a single rock face or a pair. More on this later but for now just note that if you are aiming to make two rock faces you should leave the foil in place. If you are aiming to make just one (as shown in the photos below) then you should peel the foil off the side that you are working on.
First measure the overall dimensions of the rock face you require and cut a slightly larger piece (so you can trim it to fit it later) from the big sheet that you bought from the builders merchants. The serrated edge of a bread knife makes quick work of this whereas a craft knife tends to get jammed up.
Now insert the bread knife into the edge of the foam. The cutting edge will enable you to get it into the sheet but you should try to do the bulk of the 'cutting' with the blunt edge. The idea is that you are trying to do enough damage to the sheet that you can break it apart rather than cutting it apart. If you are aiming to get a single rock face from your sheet then you can work from one edge, twisting the knife after insertion to break chunks off. If you are trying to end up with two sheets then you'll need to work your way around the edges.
Work slowly. Not only will this be safer but you'll get a better end result if you let the foam take its time to fracture. You'll hear it creaking and groaning until it eventually splits apart.
You will now have the basic rock face but it will be covered with 'boulders' that are loosely attached to it and need to be rubbed off. (Keep these for scree slopes!) You can now carve any additional features such as cave entrances or footpaths that you require. You should also offer the rock face up to its end position on the model and trim it to fit with a craft knife.
Always test glue and/or paint that you intend to use on foam as many types will simply dissolve it. I use water based household matt emulsion paint and buy cheap 'tester pots' from DIY stores so I can mix colours to my exact requirements. Starting at the top of a rock-face I give it a wash on the edges with a watered down dark colour. (Blackboard paint has a handy green tinge to it when heavily watered down.) Then I work in the main rock colour. I apply it from the tin and after working it in I load my brush with water and return to the same area that I just painted and work over it again. I continue with this sequence of one brush of 'neat' paint and one or two of water until its all covered. The water helps the paint soak into the foam and also gives variation in the tone.
Flock, trees, bushes etc. can all be added afterwards to detail the rock-face.
It is possible to create a curving rock face by cutting slits (for a concave curve) or V's (do a convex curve) into the back of the foam. Take care not to cut right through to the rock face. The more cuts you make, the more you will be able to bend the sheet. I use 'coving adhesive & joint filler' and hold the piece in position with elastic bands until the glue has completely cured/dried (usually a couple of days to be on the safe side).
Peter warns us that "the mess from the process of splitting and cleaning up the rock-face is TERRIBLE!" We strongly advise that you do not use a bread knife that will be used later in the kitchen, and that you have your cleaning up materials ready for use before you start work. It's probably also a good idea to wear gloves and a dust mask. We think however that the end results are worth it. Here's a bigger version of that initial picture to drool over:
