An acid product, and mechanical scraper, etc? 

I am not suggesting doing this for the roof but two ways to convert insoluble iron oxide( rust ) to a soluble form that is removed with a water rinse is to use either oxalic acid ( 5% solution on the rust for about 30 minutes( can wet towels so it adheres and does not run off) in water or a commercial product called "Iron Out" which is sodium hydrosulfide I think.

Consider having it etched - either acid or sand-blast - to change the appearance.If you MUST paint, please use some neutral color - nothing which stands out.

If stain remains, apply rust remover or oxalic acid solution. 6.

Acid wash.  

Boric acid is what you want for residual roach killer. 

I guess depending on the numbers, you might want to have a cleanup crew come in to vacuum them up, maybe open certain areas, to vacuum.To kill them and eliminate them, go to a hardware store, or home depot has it cheap, get a container of boric acid.

Then we hosed it down and used some acid.

But it also gives leeway to the honest people to do what is needed to do the job right without throwing out the baby with the bath water.BTW, you do know that the "Chinese Drywall" problem encompasses several brands, and that the amount of sulfuric acid off-gassing varies not only by brand, but vintage.

I was going to hire a pro to acid wash it that way none of the detail is lost since this is sand stone from Scotland and I could give all the history and the cost of this place was crazy.