If the clay was picking up so much grime - I would even attempt to clay it, I would spray the car down with a hot solution of mild surfactant and wet it every 2 minutes with a mist then spray it down.
I was awarded a similar like this to clean up about a year ago - it was my aunt (khala) old Lexus GS430, that car was in similar state - the paint seemed to be like barnacles stuck to it. A dose of household dishwash soap loosened up 70% of the junk on it. and the rest was removed by clay - I had to throw away the clay after that. Then I got my freinds who are genuine car detail nuts (100% jenyan fanatics) to buff (DA) it clean, I then put 3M glaze on it and finished off with collinite wax. Not super awesome unicorn rainbow job but 95% better than the stupid detail shops around here who think a year old wool pad on a rotary with some generic polish is "doozi quality"
