Anyone know what this discoloration on my spa walls is?

[Chongmonster]

Hey Guys,

My spa walls have a brownish discoloration on the plaster. It is a 20 year old pool so I am not sure if this is just normal. Can I empty out the spa and do some kind of wash on it to get it white?

Thanks,
Chris

 

[pooldv]

Is it rough? Could be scale. Pool School - Calcium Scaling

 

[Chongmonster]

Seems relatively smooth.