Fiberglass Pool Stain Removal & Prevention

fiberglass-pool-stains-2It only took about a month of enjoying the glistening sheen of my freshly gel-coated fiberglass pool surface before I began to notice a faint, dark ring subtly form around the water line. The optimist in me wanted to believe it was a shadow, an illusion cast by the angle of the sun.

When I realized this darkening stain around my immaculate pool wasn’t going away, I was faced with guilt-ridden questions; What did I do wrong? How could this happen? How much dirt did these kids track into the pool?

Ultimately I resolved to do everything in my power to make sure to remove this black mark on the pool’s history and vowed to become an expert in fiberglass pool stains!

Here’s what I learned along the way…

Fiberglass Pools will Stain: I really like the fiberglass pool surface. One of the advantages of a gel-coated fiberglass pool is that it isn’t porous like a plaster pool, and doesn’t become etched and rough. No pool toes, fiberglass is smooth and less susceptible to algae growth and stains because it’s super slick. Nonetheless, gelcoat can stain easily – as I found out.

Stains don’t just Happen: There are several causes of stains for a fiberglass surface just like any pool surface. Top of the list are imbalanced chemicals or minerals, high levels of metals or foreign objects can all stain the pool. In my pool, one of the main causes of the bathtub ring around my pool was oily swimmers and sunblock.

Water Balance is Important: Daily testing can help maintain the optimal water balance. It is best to have a pH balance at 7.2-7.4 and the total alkalinity needs to be at 80-100 parts per million. Maintain your chlorine at or above 1 ppm and the calcium hardness at 200 to 400 ppm. If any of these numbers get out of line, stains can develop. Simply restoring balance to your pool water’s universe is the easy way to prevent pool stains.

Specific Fiberglass Pool Stain Strategy…

pool-magicWater Line Stains: Before you ban your swimmers from applying sunblock; the bathtub ring can be removed with Clean & Perfect enzyme cleaner to dissolve it before your eyes, naturally. To prevent bathtub rings, try Pool Magic, to remove oily gunk from the pool – it really works!

You can also use Comet® cleanser and a kitchen sponge. I once spent several hours using a Mr. Clean Magic Eraser® (actually I used about 20 of them) and scrubbed each smudge until the gel-coat glistened like it had been restored. It worked well but was very time consuming, and were my arms tired!

culator-packetRust Stains: Directly scrubbing rusty or crusty stains with a Stain Eraser has worked several times for me, but this year I am going to attack the problem before it starts by adding Culator Metal Eliminator. It’s non-toxic and works on well water to actually remove metals from the water, like a sponge. Works with mineral purifiers and copper algaecides, too!

Stain Free is an amazing pool stain removerDirt Stains: If your fiberglass pool has an all-over dirty dullness, ascorbic acid is great way to safely acid wash your fiberglass surfaces, without draining the pool. Closely follow the instructions when using ascorbic acid as your pH and alkalinity may need adjustment. Stain Free is an natural Vitamin C product (ascorbic acid) that gently cleans fiberglass pools.

Organic Stains: Leaves, worms, branches. Running your filter system longer and more frequently during the early swim season while the air is heavy with pollen and tree debris will help to prevent stains from forming on a fiberglass pool surface. In addition to clean water, keep the pool as clean as possible. After heavy storms, remove the big stuff carefully, so sticks don’t get dragged around by a pool cleaner. Again, proper pool chemical balance and sufficient sanitizer is very important.

You are not alone in the world of fiberglass surface pool stains, there is help out there. You don’t have to struggle alone! 😉stains-stink!

We’re behind you 100%. Identifying the source of the stain is the beginning of recovering your impeccably clean fiberglass pool surface. Once you know the cause there are many simple ways to prevent and remove fiberglass pool stains.

Like these 20 different chemicals in our Pool Stain Removal section on our website. If you want to talk about your specific fiberglass pool stains ~ give us a call, we can swap stories!




Ryan Dornan
InTheSwim Staff Blogger



Fiberglass Pool Stain Removal & Prevention — 30 Comments

  1. We just had a fiberglass pool installed and when vacuuming all the dirt out, the pool got scratched. Is there a way to fix the scratches?

    • Hi Nancy, if they are small surface scratches, they can be blended with a fine wet/dry sandpaper. Deeper grooves can be filled with gelcoat and buffed with the pool empty. I’ve not much experience in gelcoat repairs, but it’s similar to boat or corvette scratch repair, or pool slide repair.

  2. Hi Ryan!
    We purchased a fiberglass pool we are still waiting on Concrete to be done. We had it running for couple weeks know but we have this black ring forming around water line and all the walls and sides of steps. We had the water tested they said all ok. Could you let me know what chemical would be safe to put in to clear up?

  3. I have a fiberglass pool approx. 15,500 gallon. I have a stain that can be removed by rubbing crushed vitamin c on it. I have my water tested at the pool chemical store, it has tested 0 for metals. I brought the chlorine down to 0 and my alkilinity and PH were in suggested range. I used 1 bottle of Stain Out and the stain lightened on my steps but not on my entire pool. A week later I used another bottle and let it circulate for 24 hours. No results. A month later I tried asorbic acid. I used 1.5 lbs and saw no visible results. 1 hour later I added another 1.5 lbs and let it circulate overnight,slight if any results. I do not know what to try now. Any suggestions?

    • Hi Deneice, I’m not familiar with stain out, but it may be either an acid stain remover like our EZ Stain product or a sequestering agent to tie up metals, like our Stain Away product? The Vitamin C tablet test went well, but treating with ascorbic acid (vitamin c) did not help too much? We do have StainFree Extra Strength, which may do the trick. have you tried shocking the pool with chlorine shock? If the stains are organic, oftentimes shock will remove instantly. And if neither ascorbic acid or chlorine removes the stains, the next step would be to lower the pH to 7.0 and use EZ Stain remover or similar acid to attempt a ‘no-drain acid wash’, broadcasting the material (expertly) over the entire surface areas, with the pump off. You might need 2 bottles for the average size pool. Brush the pool after a few minutes, very thoroughly and raise the pH back up after 4-8 hours or overnight, and turn the pump back on.

  4. I have a white fiberglass pool that turned dark brown. You can rub it off but takes some scrubbing to do so. What can cause that?

    • Hi Crecencio, likely an iron issue, if you have iron in the water, it can come out of solution and deposit on the walls. I would try using our EZ Stain remover product, or something similar, following instructions closely, and then treating with a Metal Free type of sequestering agent to keep it in solution. You can also have your water tested for metals, or buy a test kit of your own to narrow down the metal, but it’s usually iron when brown or red.

  5. Hi! I am a Realtor and my clients are considering purchasing a foreclosure that has a Fiberglass pool. We live in Yuma AZ where it is currently 114 degrees. That means that the pool went Green while it was in foreclosure status. It is now cleaned up but heavily stained. do you have any recommendations on stain removing? I wondered if it is similar to a bathtub, can it be drained and then put soft-scrub with bleach on it and let it sit for 24 hours? Any suggestions you can offer would be appreciated! 🙂

    • Hi Michelle, must be awful for the buyers. It is similar to a bathtub, with the exception that there could be risk in completely draining a fiberglass pool, due to unforeseen and unseen problems underground, or some flash flooding, etc. Not that you can’t drain it, but does contain risk. I would first try to make sure that the pool water is impeccably balanced, and truck in water if that was necessary, replacing most of the current water, to have excellent water balance. Then you could treat the stains, with water in the pool with two types of acids that are just sprinkled in. StainFree or our A+ Stain remover are ascorbic acid, great for fiberglass stains. EZ Stain remover is another acid that will target other types of stains. If you have ‘algae stains’ presumably, I would start with EZ Stain. If the stains are suspected to be mineral/metal stains, from aggressive shocking, start with A+ Stain…. The Softscrub would work if it was live algae, but likely it is not…good luck!

  6. Hi, We purchased a house that already had a fiberglass pool installed at the time of purchase the water was clear but after swimming in the pool the next day we found a dead gopher in the pool and the water was a dark greenish brown color. We took a sample of water and had it tested and the alkalinty was at 200 ppm, the PH was at 7.8, cynuric was at 40 ppm. So i was told to shock it with 1 pound of unstabilize tri-chlor. After adding the tri-chlor we were told to add 1 gal of liquid chlorine 2 days later. The water then turned into a reddish brown color and was told to put 2 pounds of sodium dichlor then brush the pool then add a pint of muratic acid then they told us to add calcium hypochloride 2 hrs later then clean the cartridge filters. It has been 4 days now and the water still looks reddish brown but when we brushed the pool it still doesn’t come off could it be that our fiberglass pool is stained now. What can we do to fix this? Thank you

    • Hi, it sounds like the shocking of the pool, with a high pH caused metals (iron) to come out of solution, and turn the water that color. The solution is to get the metals back into solution, nice and quietly – shhh! Use a sequestering agent, like jack’s magic blue stuff, metalfree or stain away, to lock up minerals in solution. Before adding however, lower the ph and alkalinity, with pH decreaser, to get alkalinity below 150 ppm, but keeping pH 7.2-7.6. After the sequestering agent is working, you may have some stains remaining, as you mention you have now, Jacks’ Magic Stain Solution #1 (the Iron/Copper Stuff) can be used, or EZ Stain Remover or StainFree or our own A+ Stain remover could also be used. Alternatively, if you can drain most of the water and refill with fresh water of a lower mineral/metal content, that may also ease the entire process. You may be on a well, which is common to have high mineral/metal content. There are pre-filters we sell, attached to a hose, for filtering out minerals/metals and other gunk. For pools with metals or hard water problems, a sequestering agent should always be used, in maintenance doses (every week or two), because it degrades in 1-3 weeks time, and replacement protection is needed. If you are not on well water, and have iron/copper issues, some public water systems do have high mineral/metal content, or there may be another problem like a copper heat exchanger or pipes, iron headers or pipes or steel ladders and dive stands, putting oxidized metals in the pool…

  7. Good afternoon. We have a Viking pool that was installed 4 years ago. This year, we’ve experienced scaling. We have been using Scaltec to help resolve with little results. The rich pacific color is diminished. What can we do to resolve?

    • Hi, first be sure that your pH and alkalinity are on the low side, 7.2-7.4, and 80-100 ppm. Calcium hardness levels should be 150 ppm minimum.

      You could do some regular LSI calculations – using the Pentair Langelier saturation index – and adjust your chemistry away from the scaling range, to the corrosive range, and brush daily. Having slightly acidic or corrosive water conditions and heavy brushing can help remove the scale, but it can take time. Use a sequestering agent like ScaleFree or Stain Away concurrently, to lock the minerals into solution, once they are removed from the wall.

      Alternatively, you could lower the water a foot or so and test an acid wash, by pouring a 50/50 mix of acid/water, from a flower watering can, along the edge of the wall. Rinse within 60 seconds, thoroughly. If it removes the scale (repeat treatments may be needed), then draining and acid washing the entire pool may be the ticket.

      Following treatment for removal, keep your water balanced, and always use a stain & scale chemical, to keep calcium and other minerals/metals, in solution, so they won’t be tempted to deposit on pool surfaces.

      • Thank you for the advice. One last question, is it safe to drain a fiberglass pool. There seems to be several myths out there and I want to be sure the pool or decking would not be damaged. I would not attempt this myself, rather hire a professional service.

        • Hi Michael, yes it is safe, but can cause trouble. I have drained many fiberglass pools and never had a problem, but the risk does exist that without the water in the pool, the shell could shift or raise up slightly, or weak walls could bow inward. Key is to pump the water far away from the pool, and avoid very wet weather periods. Problems with draining a fiberglass pool are rare, probably less than 1%. Hiring a pro is best, assuming they have the insurance policy to cover any problems.

  8. I have a resurface fiberglass pool that was installed back in 2007. I have aprox 24k gal like kidney shape, 3.5 feet shallow and 9 feet deep. Per my pool guy my gel coat is coming off in the some spots in the shallow area. Those areas look like light green stain, I would have thought they were algae but my pool guy says they are fiberglass exposed and they look like that. I recently installed a DE filter as my upgrade from cartridge type. I have tried brushing them with both rubber and normal brush but they wont come off. I was told brushing them will cause more gel coats to come off. I use a classic kreepy krauley as my daily cleaner. Sometimes I wonder if the Kreepy Krauley is a culprit. I would like to hear your opinion. I can probably post pictures if needed.

    • Hi bernel, The Kreepy could be a question, if those areas are bulged or raised, or a depression of just the right shape, the Kreepy could get stuck, and suck and suck on the area, but you would probably notice that if it was happening. It could be a delamination between the fiberglass shell and the gelcoat, which may be under warranty, perhaps. The gel coat is the same stuff used on pool slides, boats and fiberglass cars. A shiny ceramic like gel is sprayed onto the fiberglass, about 2 mm thick. It does wear away over time, but not usually so fast (10 years). The stain could also be caused by a ‘chemical burn’, from tablets, shock or pH down (something caustic) laying on the floor in those spots? I may not be too qualified to help, as I have not a lot of experience with fiberglass, but you can send pics to, and I should receive.

  9. I have a pretty blue ring at the water line and what appears to be blue splashes on the steps. What is cause and remedy?

    • HI, I’m not sure, but could it be from copper algaecide? Pool Party blue pool dye? Not many things make a blue stain, but sounds like something got in there. I would lower the water level and apply TSP with a scrub brush. Maybe automotive rubbing compound also would rub it off. A paste made from clorox and water, rubbed in with a scrubber sponge may help. You can’t be too harsh with the gelcoat, but an acid would also be a possibility. You can scrub with vinegar, followed by baking soda. You can also use granular ph decreaser (dry acid) or EZ Stain Remover, in a sock, and with gloves and water, scrub the wall with the stain sock, and the steps, or just set it on the steps underwater for 30 seconds. Ascorbic acid can also be used, in products like StainFree and A+ Stain Remover.

  10. Fiberglass pool : added liquid chlorine: recently added all new water: plaster now has developed black staining thruout : added metal out when pool was refilled recently : ph a little low added soda ash: get rid of black staining: add more metal out ? Perhaps raise ph a little more ? Add stain out treatment? Small pool

    • Hi Dale, what may have happened is copper or other metal dropped out when shocked. That has happened to me on pools I was overdosing with copper algaecide, and a sequestering agent like Metal Out or Stain Away, can help prevent it. I also stopped using copper algaecide. For your pool, test for metals if you can, to see what you are dealing with. If the black stains responded to the Metal Out, I would keep the pool on a maintenance dosage for awhile, usually just a cup or so per week, because sequestering agent bonds can weaken in certain conditions, and shocking the pool can knock the metals out of solution (again). If there are still stains, you can try EZ Stain Remover, a granular acid.

  11. Hi. I have a fiberglass inground pool. A few months back, I got sick. When I returned home, the water level evaporated to half it’s normal fill. I’m guessing that combined with our high calcium water levels, left heavy scale on the bottom of the pool.
    I’m wondering would the Vit C fix, work on the scale? The reason I’m asking, I’m looking at the NUVO water softener system to use in my house to control scale. It stops and reverses scale (supposedly). The system uses Vit C. Seems similiar. Thanks.

    • Hi Dave, Vitamin C or ascorbic acid, is an acid, so it can effectively remove scale by dissolving it – Stain Free and A+ Stain remover are two ascorbic acid stain removers that work on many stains quite well, and should have some effect on the issue you are having. Follow label instructions for water balance and application, etc. To test it first, you can grind up a handful of vitamin c tablets and place them on a sample area. You will also need stain & scale (sequestering agents) like Stain Away, to lock up the minerals, because dissolving them does not remove the calcium from the water.

        • Hi Ed, there are things you can do, depending on the type of stain. If it is from minerals or metals, start treating weekly with maintenance doses of a good sequestering agent like MetalFree, ScaleFree or Super Stain Away. If it is an oily, organic stains, using a maintenance dose of pool enzymes like pool perfect can prevent those stains. If the stain occurs from some outside source, or contamination of the pool, stuff getting into the pool, then you can develop a plan to reduce or eliminate, whatever is causing the stain. (assuming that you know)

  12. Hi I put in all the chemicals the pool store told me to, after having my water tested. I put in stabilizer, alkaline increaser, PH increaser. Now my white fiberglass pool is turning yellow. Any suggestions

    • Yellow algae? Does the color brush off at all? If it won’t brush off, it’s not likely to be algae. I would shock the pool, to raise the chlorine level above 10 ppm, brush well, and filter well. Yellow algae may also require a filter media replacement, recommended for yellow algae blooms, and a treatment of algaecide on a regular basis.

  13. Hi Ryan, I have a salt water fiberglass pool. Last year when we uncovered it there was a square patch on the side of the pool. It was dark grey in color we were not able to get rid of it and my husband convinced me to just live with it. This year when I uncovered the pool there are two more stains the same color but they are about a 5 in he circle one right above the other. Any suggestions what this could be or who to call? We live in Myrtle Beach SC. Thanks!

    • A circle and a square? are they perfectly round and square? that could be a clue. I’m not sure what this could be, but perhaps trying a Jacks Magic Stain ID kit may turn up something. I wonder if it may be something visible on the fiberglass shell, beneath a thinning gelcoat? Stains don’t normally form such shapes, in isolated locations, especially the wall.

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