A rebar stain in a swimming pool or spa is a serious issue.
Rusting rebar will continue to corrode within the pool structure until all corrosion is removed. Although the bleed-through stain may be small in size, the corrosion beneath the plaster (in the gunite) is always larger in scale (see photos below for examples).
Left untreated, rusted rebar will eventually rust the entire pool. There are two known methods to repair rebar stains:
1- Underwater (without draining the pool, using scuba gear and special tools), or
2- Dry (drain the pool and expose the rust with chipping hammers)
Rebar stains occur in swimming pools and spas when the rebar is not properly encapsulated with gunite (or shotcrete). Lack of gunite coverage over rebar and/or poor workmanship are common causes of rebar corrosion.
Here is an excellent article by pool engineer Ron Lacher about gunite workmanship issues.
Customers frequently ask if resurfacing the pool will solve the problem. No, it will not.
If the pool is replastered without removing the corroding rebar, the stain will work its way through the new plaster causing a new stain in your new plaster. The corrosion must be removed!
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Make sure and check my blog for articles, videos & recent photos on Rebar Stains and Bleed Repairs.
Here are a few of my rebar job photos and videos: