following up an amicable debate with modeling friends about "materials to build layout landscapes":
1) The "traditional" materials: gypsum, papier-mache, paper, vinyl-based adhesives
2) Expanded polystyrene
3) Composites: polyester- and epoxy resin, fiberglass, and similar stuff
Advantages of "traditional" materials: they are inexpensive, cleaning their residues is relatively simple (with water and soap). They are very effective for the "final" (external) layer of the landscape, where applied onto an existing structure. We have friends who are able, with old newspapers, gypsum, and vinyl-acetate glue, to build up spectacular finishes.
Disadvantages of "traditional" materials: they are structurally weak and prone to cracking, sensitive to moisture (sometimes, storage conditions of layouts are less than optimal). Drying, especially in the case of thick maper-mache parts, takes eternities.
Advantages of XPS: relatively easy to model, lightweigth. We have friends who are artists of hot-wire cutting of XPS, and really able to create very good stuff from blocks of the white material.
Disadvantages of XPS: cutting a block of XPS generates cubic meters of white fragments. As a rough evaluation, the waste volume is the same as the volume of the final "mountain" - 50% of material is scrapped. Hot-wire cutting generates fragments, but sanding/sawing XPS generates cubic meters of electrostatically-charged dust , which sticks to everything and resists to vacuum-cleaning. Moreover, the structure is solid, and carving out passages for wires/etc. sometimes difficult.
Advantages of composites: the structure is hollow, very strong, very lightweight. Curing time from resin to the finished part is defined.
Disadvantages of composites: playing with resin and fiberglass is a very dirty job: resin is sticky, can be cleaned only with solvent (before curing). After curing, resin cannot be removed, it is solid and insoluble. Cutting fiberglass cloth generates irritant dust. It is always recommended to use industrial-grade gloves, eye protection, and overalls, and lining the floor with plastic film.
In our particular case, we have extensively used composites. The larger "hills" of the layout are made with fiberglass cloth draped onto plywood formers; the smaller "hills" and the structures at ground level, soil, etc. are made of Kraft paper infused with resin.
Just to have an idea, herewith some photos:
Plywood structure in position, plywood formers to be installed - the structure is ready to accept the layer that form the "soil" (in this case, due to the small area to be covered, it was Kraft/resin composite)
Structure is ready to be cover, module is aligned with the previous module, in order to align the landscape in construction with the preceding landscape.
The covering has been draped onto the straucture, infused with resin, and, after curing, the piece is detached:
The structure of the soil is applied: the components are some wrinkled Kratf paper, sand, pigments, and eposy resin:
The finished part with the applied soil and ground texture is ready:
The construction continues with the subsequent module no. 4, which is aligned with the previous module (with a plastic foil as "release" separator from the contiguous parts) - The sequenc e of operations is the same: landscape base, first resin coat, structure of the soil, final resin coat, finishing with pigments . Note: an excess of pigments and sand must be used, in order to adsorb all the excess liquid resin; if any "wet" spot appears, it must be re- coated with excess pigment 8otherwise it appears as a glossy "plastic" spot.
Comparing the composites with more "conventional" methods, the time spent in preparation of the work is considerably long. Before starting to actually make the part, it is necessary to have all the pieces into position, resin, brushes, and other components ready.
When the actual work starts, everything happens quickly. A typical long-curing infusion 2-component epoxy resin starts polymerizing as soon as the 2 components are mixed, and has a"pot life" before gelling, of approximately one hour. In one hour, before the resin crosslinks, the parts must be infused. Immediately afterwards, the structure of the soil must be applied (note: subsequent application of resin onto previously hardened resin parts has a poor adhesion - in case of very large parts, it is better to coat a smaller section, then prepare another batch of resin and continue coating another section, blending the second coat with the non-completely crosslinked resin of the first section, and so on).
Once the resin gels (one hour), there is no way to re-arrange things. The time to complete crosslinking of the resin is approximately 24 hours at room temperature. After curing, the pieces are surprisingly resistant.
The subsequent "details"of the landscape (small rocks, bushes, trees, etc.) are subsequently applied ald glued with cyanoacrylate adhesive, or 2-component fast-curing epoxy adhesive. Final nuances of color can be applied by spraying a thin layer of nitrocellulose lacquer ad dusting pigments onto the wet lacquer.
Again, time to preparation of a composite is a bit long, but then, everyting goes on very quicky. Cast-molding-grade epoxy resin can also be used as structural material for artificial rocks, with the same silicone molds usually used with gypsum.
In the end, there is no "universal" solution for material. Some of our modeling friends like to toil over square centimeters of landscape for hours, and abhor the idea of a composite that quickly freezes in to its final shape. Someone else enthusiastically sands huge blocks of expanded polystyrene, filling their homes with tons of XPS shavings (this seems to have been one major factor in model-railroading-induced divorces...), and loath the idea of playing with sticky resins. Someone else uses hybrid solutions, such as composite- or resin-based finish onto XPS.
With regards to this latter issue, one final consideration: epoxy resins are compatible with XPS. Conversely, fiberglass-grade polyester resin dissolves XPS.
With the best wishes from Italy
Mario & Bice