The piece in the model shop had a feature I tried to emulate, though not very successfully — interlinking ends, for joining long runs of the material. Mine are too blobby and uneven to link seamlessly; if need be, I'll be better off just modelling the interstices manually. However, in the scale I'm working in (15mm) I doubt that I'll often need more length than I have here.
On the left is just PVA glue, with the addition of a bit of colouring, brushed into the mould. It's been backed with medical gauze to give it some strength. It's very flexible, which is good, but the silicon rubber repels it (it's hydrophobic) and so as well as having a patchy coverage, the moulded face is covered in pin-prick bubbles. A heat-set plastisol would probably be a better option, if I can find some in a decent colour, or which I can colour myself.
On the right is an acrylic filler (Selleys Permafill), pressed and smoothed into the mould with a plastic card. Once cured, I've glued a piece of newsprint to the back, again to give it some strength, as the filler on its own has very little mechanical cohesiveness when laid down this thin. It's not nearly as flexible as the PVA, but it will still follow a reasonably tight curve without cracking.
Overall, I'm pretty pleased with the results so far.
A few hours later...
They'll do fine, I think. The stonework skins are glued either side of a piece of mounting board, except for the curved pieces which are self-supporting (just glued back to back). They're mounted on ice-block sticks.
The PVA skins aren't really all that great though; the multitude of tiny bubbles makes the actual blockwork a bit indistinct. The Permafil skins are better as regards detail, but do need to be sealed with acrylic gesso before painting.
Making walls is easy enough, though for large-scale production it would be better and more efficient to make a few master walls and mould them complete, rather than assembling them like this. The real reason I made the texture mould was for cladding broad areas.
This little shed is built up from a sample of an architectural modeling card that I got a long time ago. It's firmer than foamcore, but less dense than cardboard or MDF. The sample piece I got was about 2mm thick, as far as I can recall. The black card of the roof is a piece of mounting board.
I used the Permafill texture with newsprint backing for this, and it went on as easy as pie, glued down with PVA. It cuts clean and easy, and it's thin enough not to distort the overall size of the model too severely. The corners are still an issue, but nothing a little filler won't take care of.