Staining: repair options for metal, organic, and scale.

Pool stains are repaired according to what caused them, and the first step costs almost nothing: hold a vitamin C tablet against the mark for a minute. If it lifts, the stain is metal. If a chlorine puck lifts it instead, it is organic. The treatment that follows runs $30 to $150 as a DIY chemical job, $200 to $600 professionally, or $400 to $800 for an acid wash.

The three stain families

Almost every pool stain falls into one of three groups, and each has a different mechanism.

Organic stains come from leaves, algae, and debris sitting on the surface. They leave a tan or brown mark shaped like the thing that caused it, and they respond to oxidation.

Metal stains come from iron or copper in the fill water, in a heater, or from corrosion. They leave brown, blue-green, or grey marks, often in streaks or at the waterline, and they respond to ascorbic acid.

Scale is calcium deposit rather than a stain. It looks like a white or grey film, feels rough, and responds to a scale remover or mechanical removal.

The identification test

The tablet test is the most useful five minutes in pool maintenance. It distinguishes the two most common stains before any chemical is bought, and it costs a few cents.

Treating organic stains

Organic stains are oxidised debris. Shock the pool, brush the area, and keep the chlorine at the high end of the normal range for a few days. Most organic marks lift within a week.

For a stubborn mark, a chlorine puck held against it for a minute will usually lighten it enough to confirm the diagnosis. Do not leave the puck in one place on plaster, because it bleaches the surface and creates a white spot that is permanent.

The prevention is circulation and brushing. Organic stains appear where debris settles, which is where the water moves least.

Treating metal stains

Metal stains need a different approach: ascorbic acid to reduce the metal back into solution, then a sequestering agent to hold it there.

1. Confirm the stain with a vitamin C tablet.
2. Lower the chlorine to near zero before adding ascorbic acid, or the acid is consumed oxidising chlorine.
3. Add ascorbic acid at the label rate and circulate, brushing the stained area.
4. When the stain lifts, add a sequestering agent to keep the metal in solution.
5. Bring chlorine back up slowly over the following days.
6. Test the fill water for iron and copper, and treat it before it enters the pool.

Step six is the one that prevents the repeat. A stain that returns within weeks means the metal source is still active, usually the fill water or a corroding component.

Scale and etched surfaces

Scale is calcium carbonate deposited on the surface. It forms when pH and calcium hardness are high, and it feels rough rather than looking like a mark. On tile it can be removed with a pumice block or a scale remover; on plaster it usually needs a drain and a light acid wash.

Etching is the opposite problem: the surface has lost material and the stain sits in the hollow. An acid wash makes etching worse, because it removes more cement. If the mark is rough and the plaster is chalky, the answer is a resurface rather than a treatment.

Scale and etching are often confused because both change the feel of the surface. Scale sits on top and can be scraped off with a fingernail; etching is a hollow that the fingernail drops into. That single test separates them.

What professional treatment adds

A professional stain treatment at $200 to $600 is mostly the same chemistry with better diagnostics. The contractor tests the water for metals, identifies the stain class, and applies the treatment with the circulation and filtration set up to remove the residue.

The value is in the identification and the follow-up. A metal stain that is treated without a sequestering agent redeposits, and a surface that is treated without finding the source stains again. A contractor who tests the fill water and the heater is doing the part that prevents the repeat.

Six steps to fix a stain

1. Photograph the stain and note its shape and colour.
2. Run the vitamin C test.
3. Run the chlorine puck test if the first is negative.
4. Treat according to the result, not according to the label on a bottle.
5. Brush and circulate for a week before judging the result.
6. Find the source, or the stain will return.

Step six is the difference between a repair and a subscription. A stain that comes back is a symptom.

The short version

Test first, treat second. Vitamin C for metal, chlorine for organic, and mechanical removal for scale. The chemical route is $30 to $150; an acid wash is $400 to $800 and does not fix etching. The prices by treatment are in staining: what a fix costs, the prevention routine is in staining: how to prevent it, and the wider surface assessment is in signs pool needs resurfacing.