Its very early days yet but I have found a mehod of getting the right kind of effect, its not complete yet and of course the finishing will blend the two together better. But for now i think I am on the right track.

If you study the pattern the marks are quite deliberate, it changes from smoothish curves to cutting straight lines in places. It looks like there was some 'Dahli' type art going on. The key I think is to avoid even coverage, let the marks thin in places and no sharp edges at all. You have to imagine being the guy with the brush and slapping it on unevenly with whatever crude tools he had. It looks to me that it may have been done with rag or some crude spray type arrangment.