How to Remove Stains in a Gunite Swimming Pool

Lead Image for How to Remove Stains in a Gunite Swimming Pool
  • 4-6 hours
  • Beginner
  • 20-40
What You'll Need
pH Testing kit
Tap water
Muriatic acid
Polyester-nylon blend bristle brush
Pumice stone
Rubber gloves
pH Minus solution
Mineral sequestering agent
Gunite plaster patch material
60 and 120 grit sandpaper

Stains in your gunite swimming pool arise from 2 main causes: calcium deposits and rust. Calcium deposits leave white lumpy stains on the gunite lining, while rust leaves reddish-brown stains around the pool's filter outlet basin, or at the pool bottom. While both types of stains are preventable, follow these directions to remove existing stains and learn how to prevent their formation in the future.

Step 1: Identify Locations of Calcium Stains

Find out where around your gunite pool the calcium stains are located. They will usually be near the top edge or just a few inches below the waterline.

Step 2: Drain the Pool to Expose the Calcium Spots

Drain the water out of your pool to expose the calcium deposits so they can be removed.

Step 3: Prepare the Muriatic Acid

Follow the directions on the muriatic acid container, which generally specify 2 parts of muriatic acid with 1 part of the water in a plastic bucket.

Step 4: Apply the Muriatic Acid Solution

Put on rubber gloves and dip a sponge into the muriatic acid. Apply it to each calcium deposit, rubbing it into the spot well. Scrub the calcium deposits with the polyester-nylon blend bristle brush.

Step 5: Scrub with Pumice

Scrub the calcium spots with a dampened pumice stone to remove any lingering mineral deposits.

Step 6: Prevent Calcium Build-Up in Your Pool

Maintain a calcium hardness level in your pool water of under 350 parts per million. Keep a balanced pH level of between 7.4 and 7.8 which is slightly alkaline. The mineral deposits may form again, but they will do so very slowly in the water at this pH level.

Step 7: Cleaning Rust Stains Off a Gunite Pool

Once you have noticed rust stain formation, you need to apply a specialized solution called pH Minus to your pool water. The granular formula of pH Minus will sink to the bottom of the pool and work on the rust stains. If you have rust stains at or just below the waterline, put on rubber gloves and shake some pH Minus into your hand. Rub it on the rust stains, and scrub them with the polyester-nylon brush you used to clean the calcium spots.

Step 8: Preventing Rust Stain Formation

Rust tends to form in pools where the pH level is toward the acidic side, that is, below 7.0. Test the pH level of your pool water regularly and keep it in the range of 7.4 to 7.8, between neutral and alkaline. Add a mineral sequestering agent to your pool water to prevent iron precipitating out of the water at the bottom of the pool.

Step 9: Patch Your Pool to Cover Entrenched Rust Stains

If the rust does not clear up after these treatments, you will need to patch the rust spots. Drain the pool entirely, prepare a gunite plaster patch and apply it to the stained area with a trowel. Level the plaster patch and allow it to dry, then sand it to a smooth finish.