Marcite Pitting In Spa When Opening Pool

[mmcwhorter]

Hello and Happy Spring Opening to All:

I have posted about this issue before, and here we are again.....the old thread was discontinued so I'll create a new one here:

I have noticed over the last four years (including recently when I opened my pool) that our "raised overflow waterfall spa" surface on both seats and vertical walls is very pitted when we pull the cover off in Spring. 2000 gal capacity approx. The rest of the pool 22,000 gallons is smooth. Last fall I removed almost all the water in the Spa except about 12 inches. When closing I drop the pool 4 inches below the tile and use a pump as needed when it rains thru the mesh cover.

When I opened yesterday, the spa was about 2 inches below the tile line...having filled with rainwater over the winter.... which is exactly my plan so I do not need to place a pump in the spa (only the pool) to keep the tile from contacting ice. So far, so good.

When closing in late Fall I take good care to balance the pool & spa.....and then SLAM. The water is very clear in the Spring. When spa water was tested at the opening I saw the pH was off the charts high, deep purple (as usual) ....the FC/CC was 0 of course. I did not test for other parameters, just added a tiny bit of MA acid to get to 7.4 and a couple of sprinkles of 73CAL Hypo. Cleaned out a little debris and fired up the pump / filter to circulate the spa. The water looks great. FC=4 and pH around 7.4.

A couple of days later I got in the spa and noticed the pitting on the marcite was about as bad as ever. Mama wasn't too happy, ruins her bathing suit..... This is now the fourth year of this situation...so I thought I'd reach out to the Forum again. Last two years I used a Lava stone to flatten out the rough peaks in the pitting. I am now going to try a light wet sanding block with a fine grid of 220 or higher...as the Lava Stone is ground up quickly and is gone in no time.

So, I am asking: is there something I should do before closing each year? During the winter?

Two theories;
I am wondering if because the rainwater has a very low calcium content that it causes the pitting when it slowly fills up over the winter? Does this make sense?

Or is it due to the super-high pH measured when I open? This is discounted because the pool has the same super high pH and it does not pit. And becuase the rainwater contribution to the pool is much less in terms of the percentage of rainwater added to the total existing pool water.

Ready to break out the wet sander block!

Puzzled. Is there a solution to stop the pitting over the winter months?
Thanks!

 

[ajw22]

Sounds like scale to me. Does your plaster have a sandpaper feel to it?

High pH can cause scaling regardless of the CH level in the water.

To me "pits" are divots or depressions in the plaster.

I think the reason your pool does not scale is the large volume of water maintains a lower water temperature then the small body of water in your spa. High water temperature with high pH raises water CSI into the scaling range.

 

[mmcwhorter]

For sure it's like rough 20-grit sandpaper....but there are many "high points" too.....so if the High pH theory is correct...I suspect next winter I need to add some heavily diluted acid occasionally as rainwater fills the spa, correct? Or, Plan B: I might as well get a second pump and leave the water inside the spa only 2-3 inches below the tile...... because I have been usually taking 90% of the water out of the spa at closing it would be difficult to calculate the amount of acid to add w/o overshooting.

Is this "scaling" damaging the Plaster in any way? Or is it OK to just wet sand it off with super fine 400 grit?

 

[ajw22]

For sure it's like rough 20-grit sandpaper....but there are many "high points" too.....so if the High pH theory is correct...I suspect next winter I need to add some heavily diluted acid occasionally as rainwater fills the spa, correct? Or, Plan B: I might as well get a second pump and leave the water inside the spa only 2-3 inches below the tile...... because I have been usually taking 90% of the water out of the spa at closing it would be difficult to calculate the amount of acid to add w/o overshooting.

I think your choices are to pump water out of the spa during the winter or deal with any problems when you open the pool and spa in the spring.

In 20 years I have had scale problems in the spa on opening once. And that year there were a bunch of reports from others in the Northeast with similar problems. I have not even looked at my spa this month to see what condition it is in.

I think climate change has brought warmer winters and some winters you get the right chemistry and temperature in the spa to brew the scale. Other winters you don't.

Is this "scaling" damaging the Plaster in any way? Or is it OK to just wet sand it off with super fine 400 grit?

The scale and smoothing it does not damage the plaster.

I have not sanded it. I drop the CSI in the spa to below -0.6 and let the water sit for a while and melt off the scale. I brush the spa surfaces with a wire pool brush while the water is idle and not circulating. Then I run my spillover and dump the brew into the pool. One or two treatments like that has cleared my spa surfaces.