Some pools show brown – reddy stains on surface. These may be caused by leaves, hair clips or similar. A light sand will see them disappear in which case they are of no further interest. If on sanding they get more intense then its most likely they are the result of rust forming on the steel reinforcing within the concrete shell. They then need to be dug out and exposed so as you can successfully treat the corroding steel. Here is an example of the surface stains and how they were dug out to clean concrete, treated and then to be refilled. Full details are in the Surface Preparation and Application Notes.
