Well, after spending more than a month doing this:

http://www.poolforum.com/pf2/showthr...hlight=mystery

I've gotten to the point where I can't get any further, but the vast majority of the algae problem has been addressed. Or is it COMPLETELY addressed and I'm being fooled?

Yesterday, after the umpteenth brushing, I noticed a really dark spot in the fillet where the bottom joins the side of the pool. I dumped about a quarter-pound of 73% Cal Hypo right on the spot and let it sit overnight. It didn't improve the next day, so I got in the pool with the scrub brush and discovered that it's actually CONCRETE showing. No plaster. It's about the size of a playing card (but irregularly shaped, of course) and I can feel the edge of the plaster taper to rough concrete aggregate.

What I presumed late in the cleaning process linked above to be residual algae or metal staining, I'm thinking now is actually the shotcrete telegraphing through thin plaster.

I'm in the Dallas / Fort Worth area, and my pool was built in August '09, so after this summer, it'll have completed its 12th year.

I must admit that a new plaster job would feel good to the feet...

Is this par for the course? A 12 year old pool needing a replaster?