Hugh Farey writes in a comment:
The ‘shroudological’ concept of what a painting is, and what might constitute evidence for one, has changed a lot since the 70s. Why this should be I’m not sure – possibly because ‘Science’ trumped ‘Art’ when it came to authority in those days, and the scientists involved in the Shroud showed little evidence of knowing much about painting. There was much talk of ‘a painting would seep through the cloth and be visible on the back’ and ‘a painting always shows the directionality of brush strokes’ and ‘a painting always has shadows which show where the light was coming from’ and even, ‘a painting always has outlines,’ all of which seem rather naive, and fairly obviously to anybody who’d actually visited an art gallery, simply untrue.
They were on better ground in the search for pigment, although even here, they did not really know how much pigment could be sufficient, so that their arguments were not about whether there were any iron oxide particles, but whether there were enough to create an image. McCrone thought there were, and produced at least one experiment which appeared to demonstrate it. I do not know if it was challenged by any counter-experiments showing the opposite.
The scientists were on even better ground in their search for a colourless binder that would hold the pigment to the cloth. This, it could be reasoned, might remain even when most of the pigment had rubbed (or been washed) off. According to STuRP (Schwalbe & Rogers), McCrone’s chemical test for a proteinaceous binder (amido black) shows positive for any linen and could not have identified anything on top of it, while their own tests were more specific and definitely ruled out any protein on the image area. However they also ruled out any possibility of starch being the binder, a finding that was later retracted by Rogers, who decided he could find some after all. This suppported his ‘starch and saponin’ surface layer hypothesis, but could also support McCrone in his search for a binder.
Even lower, as it were, than the binder, would be any chemical deterioration of the cloth itself, caused by pigment, binder, or carrier, all of which had disappeared. Guarlaschelli’s painting hypothesis depends on this, I think, and chemically, has not been demonstrated to be untenable.
So, no, the painting hypothesis has not definitely been ruled out.
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