Removing calcium scale from "plaster"/cement

   / Removing calcium scale from "plaster"/cement #1  


Elite Member
Mar 30, 2017
Northern California
Branson 3520h
This isn't really a "rural living" thing, but in lieu of finding some other forum site, I figured I'd ask the DIY geniuses here first!

There's an in-ground pool at our house that hasn't been used in a long time (like, more than 15 years), and there was some pretty disgusting water (with a floating plastic cover) in it until very recently. My son wants to use the pool, and I was sick of knowing there was a swamp monster evolving there, so we used a pump to empty it (the pool is actually slightly above original ground grade and the ground here is very dry right now so there's literally no concern about it floating/cracking due to it being empty for a while) and pressure-washed the inside.

Now, the walls have some pretty significant calcium scale deposition. I'm considering ways to remove it; not only is it ugly, but it's an abrasion hazard for skin. The pressure washer (supposedly a 3000psi unit) with a turbo nozzle very gradually was able to flake some of the worst of the scale off without any sign of damage to the gunite, but it was going to take forever at that rate (I'm not talking "all day" I'm talking spend the entire month straight or more forever, like, no).

What to do?

The first ideas were to hire it out, which would be

1: Acid-wash the pool. We've called around, and nobody seems interested. They seem busy enough taking care of pools that just need the basics. I'm still hopeful we'll find someone else to do the job as I have plenty of other projects to work on and I don't like the idea of acid washing this myself.

2: Bead-blasting. We got one callback on this, and they want an arm and a leg just to do the tile line, which is the smallest part of the job. I've watched a youtube of someone doing this themselves and I'd prefer to do the tile myself than to pay what this guy wanted - but he wouldn't even quote the rest of the pool.

So it seems likely I'll end up doing this job myself (well, me and my son).

At this point we're considering using a drill-mounted abrasive like this to knock as much of the scale off hopefully quickly and easily, and then follow up finer abrasives, and though I said I don't want to acid wash, possibly finish with some muriatic acid - once the whole pool is smooth from abrasive work, presumably it won't take too much acid work and I can probably be convinced to get the final 0.1mm of scale off that way - unless it looks & feels fine without the acid in which case I'll just call it then.

   / Removing calcium scale from "plaster"/cement #2  
Try some vinegar. It may work fairly well.
   / Removing calcium scale from "plaster"/cement #3  
Without seeing the actual encrustations, it is hard to comment in detail.

Those sanding pads are great for paint. If your encrustations aren't rock solid, it should work. But they will go into the gunnite, and you may need / want to reapply a waterproofing layer afterwards. I use a 4.25" grinder and plow through things, but wear a dust mask or respirator, and glasses. Little chips come flying off.

Judicious application of acid, especially gelled acid will help, but that too will attack gunnite, so don't spray it and forget it. I might try a small area and then see what the pressure washer does to the weakened material. (My first choice.) I use citric acid or CLR for small encrustations around the ranch. Water float valves are a perennial issue with deposits.

Sometimes changing the angle of the water jet can make a huge difference in getting crud off. (parallel vs. perpendicular to the wall)

Sand blasting crossed my mind, but I don't know how bad your problem is. There are also wet sand blasters that clip onto pressure washers. Again, I would be careful of the pool wall surface to avoid damaging any waterproofing layers that you may have.

Personally, I'm impressed that you didn't have pool monsters breeding after fifteen years of disuse and that you were able to pump it out. Mosquitos flourish in any water around here that doesn't have mosquito fish in it.

All the best,

   / Removing calcium scale from "plaster"/cement #4  
I should add if you use acid or vinegar do not let it mix with any sodium hypochlorite.
   / Removing calcium scale from "plaster"/cement #5  
I have Craftsman and HFT corded DA (AKA random orbit) machines that I'd look to how body shops use stickum to attach discs to their base pads. Cheap abrasives to see what works best, trimming your own can be sloppy, look for green or blue abrasive discs/sheets.

If using vinegar or citric acid (always alone) I'd want to rinse and neutralize areas immediately after treatment. there must be a sealer that a guy could DIY once the job was done. Best of luck with whatever you try/use.
   / Removing calcium scale from "plaster"/cement #6  
If you have not contacted a DIY pool store I think I would. When I lived in Florida these pools were being filled in or replaced with new one piece pools as the upkeep was prohibitively expensive. I hope you can make this work for you.
   / Removing calcium scale from "plaster"/cement
  • Thread Starter
I bought a few different abrasive things to try and found that a 50 grit disc on my 4.5" grinder works great - takes the scale off without biting into the concrete. The discs can be gotten for $0.50 in bulk; one disc got about 8 square feet...

I'm considering getting a 7" grinder to make the job go a bit more quickly. Can anyone recommend a variable-speed unit (hopefully with a dial to set the max speed, not a variable speed trigger)? Corded is fine. Something that can take a dust shroud would be nice, if those work with sanding discs (I didn't use one on my test section but can tell that it would be helpful if possible; I'm getting a half-face respirator too as there's tons of dust and my crappy covid mask isn't going to cut it).