I'm debating whether I should take this job or not. It's a high school library that had a leaking roof and left spots/stains all over the place. I'm tempted to take on the challenge with my Cimex but I'm not sure the problem areas will come clean. What chemical or process would be recommended if I were to accept this job. Of course I want to make a great impression.

I would use Encap D/S2 or Hydrox. First get a spray
bottle and mix 50/50 encap solution and take a brush with you.
Test some of the worst spots. Only clean the center of them.
That way you will know if the bad spots will come clean. Always
pre-test on every job.

I just cleaned an office that had many water leaks in the roof. They finally got the roof fixed and had me come in and clean it. I used the cimex with ds2 and when you get to the water spots just go over the water spot without pulling the trigger at first. If need be you can use a little bit of cleaner but less is better.