20 gallons of acid???

[waterbear]

I assume that this is a new plaster pool and that the pH has been constantly high because of the curing plaster. In that case it very possibly is scale deposits and adding acid would be called for but NOT 20 gallons, IMHO. I would not think that more than about 3 or 4 gallon max would be needed. Even if your PB used an acid startup to expose the aggregate the amount of acid used in a pool your size would only have been about 10 gallons!

FWIW, depending on how the pH is being testing it is very possible that the pH has been above 8.0 since many test kits cannot test above that (and pH above 8.0 is not uncommon with new plaster).

It sounds like your pool builder wants to do a no drain acid wash but the amount of acid he wants to use does seem excessive to me. What exactly does the scale look like. Also, the 'tan buildup" in the filter--put a bit of diluted acid (dilute about 4 to 1 or so) and let us know if it fizzes and bubbles. If so it is most definitely scale. If not then it isn't. Also, the 'sand' in your pool might not be from the flagstone but is most likely plaster dust from the new plaster and it is very common with a standard startup (as opposed to an acid or bicarbonate startup).
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I would also be interested in the CH and TA of your well water. Remember, that scale is calcium carbonate so if it is depositing on the pool and filter surfaces it means that there is LESS calcium and TA in your water! Once again, your are probably seeing plaster dust in the filter and this is also common with new plaster startup.

Scale is a likely possibility with new plaster but I question the need for 20 gal of acid in a pool that size and would be concerned about equipment damage!

Finally, when you say "wet edge pebble sheen" finish are you referring to a negative edge pool with a
Pebble Sheen surface (Pebble Sheen is a product made by Pebble Tec) or are you referring to one of the Wet Edge Technologies pool finishes (a competitor of Pebble Tec and they do not make Pebble Sheen but their equivalent product is Satin Matrix)? The reason I ask is if you have a negative edge pool that would cause your pH to constantly be rising and would certainly contribute to scaling with new plaster.

Have you been brushing your pool daily? That is very important during the startup period.