CIP deterioration is most negatively affected by soil issues (not applicable here), speed of flow through pipe (faster is better, within reason), and makeup of the effluent and gases within (mostly acidic for waste, possibly basic in heavy cleaning situations).

It was non-acid stain, very easy to apply, and it looks amazing!

Batteries and battery acids are also common, dry cells and lead acid for larger models.

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.