August 26, 2022 Carney Tech

How do you fix spots, stains, and blisters in fiberglass pools?

We all know the problem of pool stains. Fiberglass pools are generally low maintenance, but eventually, something will get into your pool that doesn’t belong there. Whether it’s a bag of charcoal you forgot or grass clippings from your neighbor’s lawn, unwanted debris can cause black spots and stains in your pool. Luckily, these problems aren’t hard to fix! Here’s how you can remove them:

Fiberglass pools are, generally speaking, low-maintenance.

They’re easy to maintain and clean. It’s also easier to repair them if there’s a problem; in most cases, you can even do it yourself.

fiberglass pools

However, all pool owners experience issues with their pools from time to time.

Fiberglass pools are low maintenance and easy to care for, but they aren’t perfect. Occasionally, you’ll find yourself scratching your head when you notice a problem with your fiberglass pool. Here are some things that can go wrong:

  1. Black spots on the surface of the pool
  2. Stains on the bottom and sides of the pool
  3. Blistering or bubbling of paint

Tackling these issues may get tedious if we don’t know what caused them or how to fix them. But don’t worry, we’re here to help!

Black Spots: These are often caused by the high iron content in your water.

Black Spots

In the world of fiberglass pools, nothing is more common than black spots. Black spots can occur for a variety of reasons, including:

  • An above-average concentration of sunlight on the surface of the basin (especially in southern climates)
  • Surface dirt that has accumulated on the pool water or around jets/filters/electrical equipment and has not been removed by vacuuming or skimming. If this happens often enough, it can cause these areas to heat up and displace dyes into other areas of your pool. This is especially true if you do not use a weekly shock treatment (chlorine).

The easiest way to remove these stains is with a pumice stone (for softer stains) or an orbital sander (for harder stains). If your fiberglass pool has been stained by black algae, you should start with the pumice stone. Scrub the stain until it’s gone, then rinse your pool thoroughly. If there are still some stubborn stains left, use an orbital sander with 120-grit sandpaper instead of 220-grit sandpaper – that way you won’t scratch the fiberglass walls too much while you’re trying to remove those pesky bubbles.

Stains: Stains are usually caused by various elements entering the pool’s water like grass clippings, leaves, and dirt.


If a stain is caused by the high iron content in your water, you can easily remove it with a simple pool cleaner like WaterCo’s Trident Vacuum Cleaner. If you’re not sure what caused the stain and want to be sure it won’t come back, you need to test your water for various metals and minerals.

If you have hard water spots or stains on your fiberglass swimming pool’s sides, it could be due to several factors including:

  • Not scrubbing algae off of steps regularly (or at all)
  • Soiling caused by debris (e.g., leaves, sand) can be removed with a leaf rake or leaf vacuum cleaner and then brushed with an anti-stain gel.
  • Black stains are caused by mold that appear during the winter months when no chlorine protects the surface from mold growth; scrubbing with bleach kills any remaining mold spores so they don’t return the next year.

In some cases, they may even be caused by bacteria. If removed in time, they’re usually easily removed with a stain remover.

If the stain is still fresh and hasn’t had time to bond with the surface of your pool, you may be able to remove it with a chlorine-free bleach. You can get these at your local pool supply store.

Make sure the products you use are safe for fiberglass surfaces before you use them. Some chemicals can damage fiberglass. Follow the instructions on the container carefully – it’s important to get the timing right.

Blistering and bubbling: Both blistering and bubbling are signs that there’s something wrong with your pool’s gel coat.

Blistering and bubbling

Blisters are generally caused by water molecules seeping through the gel coat lining to the fiberglass layer creating air pockets in between. This happens due to the high acidity level of the water that slowly weakens the surface of the pool. It can also be faulty manufacturing processes at times if proper care isn’t taken (using bad quality materials or wrong techniques).

It’s not always clear what causes these problems on the walls or deck of a fiberglass pool, but it could be related to improper installation or cleaning techniques (or both). To find out if you have a problem with your fiberglass coating, look for patches of discoloration that differ from their surroundings – if so, they may be areas where the coating has been completely removed and replaced with fresh resin.

Indoor pools may have blistering if humidity levels aren’t sufficiently controlled during the construction process.

If you’ve ever seen a fiberglass pool and thought it looked like a giant bubble, you’ve your finger on the pulse of these things. Bubbling is caused by not controlling the humidity during construction. Indoor pools can have this problem if the humidity isn’t adequately controlled during construction, but outdoor pools don’t have this problem unless they’re located in an area where rainwater frequently pours over them.


fiberglass pools

If your fiberglass pool is cracking, blistered, and bubbling like a bubble bath on steroids, there is no need to worry. It simply means that the gel coat needs to be repaired by replacing it with new layers of resin-soaked fiberglass fabric over the damaged area.

Overall, the regular maintenance of your fiberglass swimming pool structures is of considerable import if you are to enjoy a problem-free time with your loved ones in your new or already installed fiberglass swimming structure.

However, if you make the prudent choice of SmartPools, our rigorous and stringent quality control process prevents all the aforementioned issues at the manufacturing level itself. Feel free to reach out to SmartPools for any enquiries or questions with regard to fiberglass pools.

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