Failed on my own... help...

[dulrey]

It feels rough, not smooth. So I'm doubting its black algae. I was just being "hopeful".

Parents got the house in 97, but it was built in 90. As far as I know, it's never been acid washed. I have several locations where this is peeking through.

 

[JamesW]

Most likely gunite.

Has it ever been replastered?

 

[dulrey]

Never in the past 20 years.

 

[JamesW]

Looks like regular white plaster with marble aggregate. Based on the age, it's normal to have spots where the plaster has worn down to the gunite.

 

[dulrey]

Hmm. Re-plastering may not be an option currently. After doing a bit of research, it looks like WaterWeld may be a good fit for the patch job. Anyone able to comment on this?

 

[JamesW]

Water weld is better for cracks. E-Z Patch 1 Fast Set Underwater or PermaPatch might work better for larger areas.

Working under water is never the best choice. Working dry is usually better. However, draining can have serious risks for the pool structure.

If you're not losing water, you can just ignore it if the appearance doesn't bother you.