If you mean calcium deposits at the water line, you can contact a professional who uses mineral salt instead of glass bead blasting. Here is a list of service professionals www.cleanpooltile.com/contactus/html
100-150 ppm of calcium in a fiberglass pool has been shown to reduce staining and spotting. Other than that, I know of no other benefit.
recakle
Not exactly sure if this is the problem, but from my experience in the pool business I've seen purple build up on fiber glass pool's surface, and it seemed to be a combination of two or more of these conditions: low calcium, low ph, and well water
I recommend that you contact a professional for this.
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Calcium. This is a serious problem in the southwestern states (AZ, CA, NV). Calcium will leave a white ring around the pool tile. If left to build up, it will require hiring a pool tile cleaning company to remove it. There are 2 primary methods of removing the calcium: blasting with glass beads or blasting with magnesium sulfate.
Improper water chemistry & neglect. It depends upon how severe the deposits are. The best way to remove calcium from your pool tile is, to sand blast it off with glass bead. Surface Renewal does this. (949)269-3122
You can use mild muriatic acid to remove clacium build up on tiles.
Baking soda will not soften (remove calcium hardness) from pool water. It will raise total alkalinity.
It will make the water cloudy. to remove calcium carbonate simply lower pH in the pool to 7.0 or lower and calcium carbonate will redissolve back into suspension and water will clear up. You can get this characteristic by adding sodium Carbonate too quickly.
I've done that with epoxy on a few occasions.
Hum, not sure if you can actually plaster a fiberglass pool. The glass would have to be removed first. You will have to consult a pool builder, a company that installs fiberglass shells or a company that installs fiberglass in plastered pools - thus converting a gunite/plaster pool into a gunite/fiberglass pool. The later uses the old pool shell as the sub-grade or foundation so to speak. k
To raise calcium in your pool use calcium chloride. You premix it in a container to let it dissolve and add 6lb for 10pmm in a 50,000g pool.