Rebar is the metal reinforcement rods that pool builders use to reinforce the concrete shell of your pool.
Rebar stains can look very different, but they all occur from the rebar rusting inside the shell and making its way out through the plaster. The picture to the right shows a piece of rebar that has been rusted through.
The repair entails chipping down to the rusted rebar and cutting out the section of rusted rebar causing the stain. Then the exposed rebar is coated with a rust inhibitor. The cavity is then packed with a high psi non-shrinking concrete and then the area is replastered.
**Because of the aging of your plaster it can be difficult to match the exact color. These patches will be noticeable no matter what you do, but the lighter the plaster the less noticeable.