Sand Paper Feeling on Hot Tub Surface – Hot Tub Scale

Has the interior of your spa developed a gritty hot tub scale, often described to feel like sand paper?  This is actually a very common problem in hot tubs, so know you are not alone.

What causes “sand-paper” hot tub scale?

There are a few different things pertaining to your water chemistry that can cause this kind of hot tub scale. First is your pH, pH in your spa should be maintained at 7.4-7.6. Warm water mixed with high pH can cause calcium to build up and settle on the surface of your spa. When the pH is high it allows the minerals in the water to fall out of solution and that is when the scale forms on the surfaces of your spa.

Calcium hardness in your water can also contribute to this type of hot tub scale. Often the calcium comes from initially filling the tub with a water source that is high in calcium.

Alkalinity is the third factor of water chemistry that can cause this type of hot tub scale.  Total alkalinity is a measure of buffers in your water that help control your pH levels. High alkalinity will normally result in high pH which can cause scale to form. Total Alkalinity levels should be maintained between 125-150 parts per million (ppm).

As these are the main causes of hot tub scale it is important that you have a water test kit that will test for these factors and that you strive to maintain balanced water chemistry.

How do I fix it?

Step 1. Adjust your chemicals to get the correct levels of pH and alkalinity. Using a water test kit check the water every 4-6 hours and adjust as needed to get pH levels in at correct level of 7.4-7.6. Having the correct chemical levels should help the scale loosen up and begin to dissipate.

Scrub the spa surface to try and loosen the scale. If the scale is fairly new it may simply brush off but for harder, firmly deposited scale you may need to drain the spa and use a cleaner like ProTeam Surface Clean and a Pool and Spa Scrubber Brush to remove the scale build up on your surface.

We recommend you do not use household cleaners or anything that does not say it is specifically for hot tub use as these items could scratch your surface or cause water chemistry issues and foaming.

Step 2. If you had to drain the spa refill it. If you are having calcium hardness issues we recommend the use of a Spa Pre-Filter at the time of refilling your spa as it can help remove excess minerals from your water prior to it being put in the hot tub.

Once your tub is refilled, balance your chemicals making sure they are all at the recommended levels.

Step 3.  Add a Stain and Scale Preventer, like Spa Pure Stain and Scale Preventer, and continue to add the recommended dosage weekly to help avoid further hot tub scale build up.

The key to maintaining a happy, healthy spa environment free of hot tub scale or other irritating water issues is to constantly maintain proper water chemistry. Test your water using a good test kit often and make adjustments when needed.
Spazazz RX Spa Aromotherapy


July 27, 2016

We had the sandpaper feel only in 1 corner at first, dip tests proved water chemistry was ok. 6.8-7.4 the sandpaper feel eventually spread across 60% of area. i used a non abrasive sponge to try and shift it (bad idea now left scratches which i mistook for scale build up. I purchased more dip tests from a different supplier and found the new one t gave me ph 7.8. where my original was showing low 6.8 so i was adding Ph+ to bring it back to balance. after reading forums i now realise it was high all along and the colours produced on the dip test where affect by my Hard water. I am wondering now how can i remove the scratches on my hot tub surface without damaging it even further. Any help appreciated.

October 29, 2016

how long is a spa pre-filter good for?

May 1, 2016

We got the sandpaper feel after we filled the hot tub. We thought it came from a filter we purchased. Could that be possible?

Kasey LaRose
May 16, 2016

The sandpaper feel is usually due to pH. Check out this article:

It’s unlikely that a new filter cartridge would cause this issue. After you fill up your spa, be sure to check the water balance and adjust accordingly. The sand paper feeling can happen very quickly if the water is out of balance. Hope this helps!

jerry branstetter
May 21, 2016

I have an older tub but good one. As i was driving over the road ir sat with water in it seldom run for a year. No less i have well water. The deposits are really bad. Ive tried the hot tub cleaner with no change at all. So ive been useing clr and green scrub pads. Im going threw the pads with one scrubbing. Getting a little at a time. Im wondering if i can use some type of buffer or dotco with a soft scrub wheel on it and clr. As it is i dont feel like i will ever get the smoothness back.

February 16, 2015

We have hard water, we have two filters before the water goes into the tub, this scale and sand paper feeling seems to be happening to our tub—-I put stain and scale in each time I add chemical—will it go way or do you feel we need to drain it and scrub it—-I suppose it will need a new filter also—-any other advice you have we would appreciate—Thank you very much—

Kasey LaRose
March 24, 2015

Sorry for the late reply. A lot of time the “sandpaper” feeling on the spa is actually due to a pH imbalance. I would recommend scrubbing the shell, so it is smooth again and then monitoring your pH closely after that. Sometimes if your pH, bromine, etc. is too far out of whack, it can also mess with your test strips. Clean the spa, add fresh water, and then test the water regularly, keeping an eye on the pH. I hope that helps!

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