Structures were coated/plastered with a combination of the following ingredients:
White glue (the wood sort)
Paint - so as to get rid of the very white colour - more paint is added after plastering has dried
Grit or flock sort of sand - thinking may want to use a finer type on the fort
poly filler - yes the sort for ironically getting rid of cracks

plaster paris
The cracking is therefore largely hit and miss - though I found by making the above mixture wetter and drying it quickly in the sun or with a hair dryer - that natural cracking occurred - which was an unexpected bonus. I suspect it is the plaster paris that caused the cracking so adding more of that and less white glue should also do the trick. I do not mix as a baker would do. I just mix as I go really with then consequently varied results, which I hope look more natural.