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How to Remove Stains from a Fiberglass Pool

How to Remove Stains from a Fiberglass Pool

Fiberglass Pool Information  |  Pool Maintenance

In my house, we take turns doing chores, but I’m exclusively in charge of laundry. My favorite (?) part is collecting all the stained clothes so I can apply stain remover and then scrub them like an old-timey washerwoman. It feels as if I should later buy hardened brown sugar in paper bags tied with twine like I live in a Laura Ingalls Wilder book.

But that’s just washing clothes. How do you feel about being an old-timey washer for your inground fiberglass pool stains?

Yes, stains—any type of surface discoloration or blemish. It might be in the pool surface or just on the surface.

Concrete pools have the worst trouble with staining. But it can happen to fiberglass pools too, although we rarely see permanent stains on our pools.

Three things you need to know:

  • The types of pool stains
  • How to avoid pool stains
  • How to remove pool stains


What kind of fiberglass pool stain is it?

All stains can be sorted into two categories: organic or inorganic. Literally everything is one of the two. Sort of like how all people can be categorized as either “pro-cilantro” or “anti-cilantro,” but a more useful dichotomy.

You can use a stain identifying kit—testing agents you add to the water—to know for sure which kind you’re working with.

Side note: We love Jack's Magic, and if (like me) you live for step-by-step instructions, they have a super-helpful identify/remove/prevent program for their products.


Types of organic pool stains

Common sources of organic stains:

  • Leaves (green or brown)
  • Berries
  • Dirt

Algae isn’t a stain, but black algae can look like black spots on the pool surface.


Types of inorganic pool stains

Metal stains

Metal stains used to be common with older, white fiberglass pools, but haven’t been an issue more recently.

They’re pretty easy to deal with. Nice.

Common sources of metal-based stains:

  • Iron (orange, brown, or tan)
  • Copper (green or blue)
  • Manganese (brown, black, or purple)

Cobalt stains

Cobalting shows up as black spots on the surface.

Fun fact: It’s technically not a stain! The water penetrates the gelcoat, reacts with the resin, and creates a black byproduct. It’s caused by osmosis, so it's like osmotic blisters but...not...blisters. Oh.


Calcium scaling

Okay, so this technically isn't staining either, but it rides the same wave.

Scaling is calcium crystals that have built up on top of the pool.

There are two types, calcium carbonate (white, flaky) and calcium silicate (white-grey).

How to tell the difference: drip some muriatic acid on that stuff. Calcium carbonate will react and foam; calcium silicate won't.


How to avoid fiberglass pool stains

As with all problems in a pool (or in life, really), it’s best to prevent stains in the first place rather than clean up the mess later.

These tips apply to all stains, no matter the origin.


Maintain your water chemistry

I know, I know, we say this for everything. That’s because it’s important!

You can prevent all kinds of pool problems by keeping your water levels in the ideal range.


Clean your pool regularly

Scoop out or vacuum dirt, leaves, and bug corpses. This helps keep the waterline from staining.

Bonus: your water will look prettier.

Clear out any foreign metal objects too.


How to remove fiberglass pool stains

Let’s say, theoretically, you have a li’l whoopsie-daisy and a stain shows up in the pool. Have a good cry (I won’t judge), and then we’ll get down to business.

Let’s go basic: see if you can wipe it off. Yes, this is the “have you tried turning it off and on again?” solution of pool cleaning. Yes, you should still do it.

Use a soft sponge/fabric and products that are specific to fiberglass pools. Using harsh chemicals or rough tools could damage the gelcoat.

If the stain remains, your next step depends on the source—the type of stain. Time to use that water testing kit we talked about.


Removing organic stains

Organic stains usually lift out on their own if you brush them.

If hitting it with a brush doesn’t work, rub it with a stain eraser. (This is not, as it turns out, just a Mr. Clean Magic Eraser.)

Stain eraser


If a brush and a stain eraser both don’t work, give it time. I know that’s not the magical answer you’re looking for.

At most, the stain should lift out within a couple months. If it doesn’t, call a pro.


Removing nonorganic stains

Metal stains

The simplest route is to rub a vitamin C tablet directly on the stain. This works best with small stains.


The electrolysis causes the metal to release from the surface of the pool.

You’ll then add the appropriate metal sequestering agent to, well, sequester the metal.

For a big stain, you can level up to ascorbic acid:

  1. Lower the chlorine to 0.0 ppm and the pH to 7.2.
  2. Run the filter and add ascorbic acid (about one pound per 10,000 gallons) to the water.
  3. After 24 hours, re-balance the water chemistry.
  4. Add the sequestering agent to the water.

Calcium scaling

You can remove calcium carbonate using a stain eraser (for quick results) or a fiberglass-pool-safe scaling treatment (added to the water, with slower progress).

Calcium silicate is a tougher row to hoe. You'll need that pool scaling treatment. If you want faster results than that, contact your local pool pros to see if they can help you out.

Want more info about fiberglass pools?

All year round, River Pools manufactures and installs fiberglass pools in Virginia and southern Maryland. We also operate in other areas of the country through our nationwide dealers.

Any questions about fiberglass pools, or even just pools in general? Get in touch with us, and we’ll be happy to help you!

Buying a fiberglass pool in 2024? GET OUR COMPLETE POOL BUYER'S GUIDEBOOK pricing, sizes, designs, maintenance, and more... CLICK HERE TO GET YOUR EBOOK NOW!

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