I have a few areas on my NOCH Cortina layout where stone or brick walls are just smooth grey plastic. I'm going for a bit more realism. I thought I could apply a very thin layer of something like latex spackle and draw stones on it with a pointed stick or dowel, like a pencil, and then when dry I'll paint it again stone grey, followed by a dark wash, and dry brushed highlights. I'd like a recommendation for what kind of paste would work best. Something that dries slow but won't run and remains slightly flexible for handling afterwards.
Woodland Scenics Foam Putty is one option and is one of the first materials I use for situations like you described. It's very lightweight, you can mold it pretty much anyway you want and to whatever thickness you want. Also you can carve rock facings into it while damp or do it once it's dry. I use a toothpick for carving the rock faces when its damp or a hobby knife to carve once dry. Once dry, you can paint the foam as you described above.
