My experience with direct acid washing, several times, on very old and then ageing plaster, 12 years after the pool was replastered, leads me to these questions and comments, albeit few.
What color is the plaster and how old is it?
How extensive is the scale? Solid or spots? Of course there will be areas that are much more or less thick.
As I have colored plaster judging just how much plaster is removed, while removing scale in unevenly distributed scale areas, is much easier to see, as the dissolving plaster comes off in a dark color, unlike the runoff of the scale, even when it is stained.
The last acid wash I did on my pool was 2008. It was around the 5th acid wash I had done on the pool in 22 years; two on the old white plaster and three on the blue plaster, applied in 1996. The blue plaster heavily coated with scale in some areas and lightly coated in other area. I didn't determine this until the first wash was done using the weakest solution. I decided to not hit the bottom of the pool very hard as it was pretty well spotted with areas of thin to no scale and some areas of very, very heavy scale, for fear of dissolving too much plaster.
Of course you know about my two hours of angle grinding, using concrete disk, on just one step.
Here's what I observed after finishing, which might be of benefit. My friend and I did the last process in two full days, probably took us 20 hours, with lot of breaks. It was 100 F outside so we had to "stay alive".
We did not directly acid wash the basin of the deep end where some of the heaviest scale was laid. During the process we allowed about 2.5 to 3' of rinse water and acid to collect in the basin. I neutralized it with baking soda and pumped it out, doing this many times, over the two days. I don't know what the pH of the water became before I neutralized and pumped out as I was only using fresh test strips. From watching one video we took, after using 6 bottles of acid solution and collecting it in the basin over about two hours time, the volume of the basin was about 2.6-3 K gallons of water and 6 gallons of acid. BTW.... I had been adding baking soda every once in a while, while this acid pool was collecting so I only needed to add about four more lbs before the pump out. So pH was not "6 gallon pH the whole time". Sorry I can't be more specific. The calculation called for 2 lbs of baking soda per gallon of 3x.x MA and I had already added 8 lbs of baking soda, couple lbs at a time, during the two hours.
At any rate, after the job was finished and upon refill there was an almost scale free bowl in the deep end of pool. I'm posting a YouTube in a bit so you can see the pool (and me, and hear dear Shelsey's voice) It doesn't appear that much excess plaster was removed.
gg=alice
Dunno, no opinion here. I was just thinking how much the PB added when we had our pool replastered......
Isn't information marvelous? :-D 
And I'm a blond after all, with "old timers" and a few wines under my belt (only way I can stand to sit still to do this indoor Crud). I'd much prefer the broom and mop to this *********************!!!!!!!!!!!!!!!!
