I was just browsing, seeing what people are doing at least at the water line and up to care for the fiberglass, if they don't have tile. I want to maybe drop the water level and try some boat wax?
Anwyay, I ran across this thread, and it got me to thinking about the problem from a non-chemical perspective -especially considering the results of the survey, and aligning that with the description of the issue you are having and why mechanical solutions seem to be the only solution.
So, we know that gelcoat is just a very specialized polymer clear coat. There are many ways to break these kinds of coatings down, but the manufacturer takes a lot of these into account with the design of the polymer, additives, etc to protect longer from UV, chemical exposure, etc. And surely these are a factor with sun and pool chemicals. My pool in particular is only 3 years old, and I have this issue, very visible when I had a small ding repaired on the shallow lounge area and we had to drain a large portion of the pool. Mine was also smooth to the touch, does not produce residue, and is unreactive to a reasonable acid wash. Once the water is back up, the "scale" vanishes and is only visible when evaporation brings the pool level back down to the typical waterline. The pool repair man for Thursday pools says he sees this in all pools he works on.
What I haven't seen discussed in this thread is long term low grade exposure of the polymer film to mechanical forces. This is happening from something isn't really given much concern for most pools under the TFP philosphy: TDS. My line of thinking is, even in solution, dissolved solids will have some form of abrasive effect on a substrate. So what if what you are seeing is not deposit, but more like crazing or abrasion marks on the surface from water with TDS. So the optical properties are changing at the surface of the gelcoat, not from deposits necessarily, but from micro-abrasions accumulating over time. They don't necessarily damage the coating to the extent that the coating begins to fail and fall off the surface, but enough that the index of refraction is changed.
These defects in the coating are hidden when the water is covering the surface since the water fills the voids and returns the index of refraction nearer to the that of water. Kind of like when you put a pyrex beaker in cooking oil and it disappears. Sanding of course removes any deposits but it would also smooth out the small abrasions and return the index of refraction back to a mean level where there is not visible difference. My experience as a chemist, there are few deposited inorganics that will not react in some fashion to a strong acid. At least those found in most pools and would come from said pool water. This leads me to believe that this process of chalking, at least in the case seen in the survey where the pool is kept within TFP balanced parameters and produces no residue, is not depostion, at least not in large part, but perhaps a combination of mechanical and U/V crazing of the polymer film.
It would be interesting to have like a small vessels coated in the same film material, maintain it with a control pure water, TFP non-salt lowest TDS possible water, typical salt TFP levels water, similiar sun/temp/water flow exposure and see how they would perform over a year.
Anyway, just my ramble. Anyone with any success in an easy to do solution for shining back up the fiberglass from the top of the pool shell to the waterline, shoot me a DM. I'm perusing the forums too, but wouldn't turn down any experienced advice.
www.troublefreepool.com
