Conflicting "experts" and The Mystery Stain


New member
Aug 10, 2020
Las Vegas, NV
Hello all,

I'm a wanna-be do-it-yourselfer trying to figure out if I should (or even can) de-stain my 1,560 ft3 fiberglass pool or paint it. Here's my dilemma:

First, what the heck is this?



This pool has been empty for about five years. I had it acid washed when I bought the property in 2007 but drained it after three years of little usage (with the intent to someday fill it in and convert it to a putting green – remarkably there are no lumps or cracks). The stain has been there since the pool was drained. It was textured and threw off a lot of powder when rubbed. After a recent chlorine wash the surface now feels smooth and clean and it appears this is a discoloration of the fiberglass itself. However, when I rubbed a spot with nothing but a wet rag I was able to remove about 50% of the upper (lightest) of the four layers which can be seen on the left of the first image (again, this was after the professional chlorine wash). But even using an SOS pad had very little affect on the middle or bottom (darkest) layers. The pool guy now tells me there's nothing more that can be done to lighten the stain*, but I'm finding so many products and methods that can supposedly remove any type of pool stain that exist (usually one product per specific stain type, but some claim their single product is a pool stain panacea). Everything I find online that looks like this stain is either on a different surface type or only explains what to add to the water of a filled pool.

So, is there anything I can rub on a dry stain of this type to remove it?

My challenge with learning how to paint it is that so far I've read 10 online articles and asked the guy down at Leslie's and five say I need a primer coat before applying the epoxy pool paint, four say I only have to lightly sand it, one claims I only need to be sure its clean, and one instructed me to get another acid wash first to "help the paint stick".

So, on a fiberglass pool in this condition, do I need a primer coat or light sanding?

And finally, I’m in Las Vegas where it's going to be over 100° literally every day for at least the next 4-5 weeks, and 112° is predicted for next weekend when I planned to do the job.

So, what happens when you paint epoxy on the same surface you could fry an egg on?

Thanks guys. I appreciate any guidance you can offer.


* But says he'll paint it $1,200.

Texas Splash

Mod Squad
TFP Expert
LifeTime Supporter
Welcome to TFP! :wave: My guess is one of two things: Either calcium build-up that took place after your first acid wash or perhaps the discoloration is from oxidation of the gelcoat after being exposed to the sun while empty. Most people who encounter gelcoat issues are not able to fully drain their FG pool, but in the Vegas Valley there's no fear of an elevated water table so you are fortunate to not have a "pop-up" situation. Having the gelcoat professionally recoated would be the ideal method to go. In our forum, we have found that painting a pool of any type is generally not as successful or long-lasting as one would hope. If you're still curious to try other methods to refurbish the finish, you might check with your local marine stores. I'm sure there are some with Lake Mead nearby. They should have some good FG compounds and polishes for boats. You might also try some 600-800 grit wet/dry sandpaper on a test area (wet though).


New member
Aug 10, 2020
Las Vegas, NV
Hello, folks. Well, here's my dilemma (which gets worst near the end)...

I recently posted some images of a huge "mystery stain" that covered most of all four sides of my pool (reattached here). This is a stain that still remained after I had paid a professional pool cleaner $400 to perform a "chlorine wash" which included the use of a wire brush to remove the crusty brown substance that covered the stain (which through off a thick powder when rubbed by hand). I was told that there was no guarantee that all of the stain could be removed but was explicitly told that some amount would but since they didn't know what kind of stain this was we would have to wait and see the results of the chlorine wash to determine if it was effective enough to avoid having to paint the pool – which he said he would do for $1,200. Well, the first wash resulted in about a 5 foot section where the crusty brown stuff had been completely removed (leaving the surface very smooth) but mostly still remaining everywhere else (other than a few small random spots). The stain was completely unchanged. After sending him a video where I demonstrated how the stain could be reduced somewhat with some scouring powder and a wet sponge, and how the crusty brown material was still covering most of my pool he sent the same person out to work on it some more. This resulted in the crusty stuff being completely removed (which proved it simply wasn't done the first time) and the upper, lightest band of staining being reduced slightly but still having no effect on the darkest, ugliest staining. After protesting that I had essentially paid $400 to have one garbage bag full of leaves removed and a thin ring of mud around the drain mopped up (which I could have easily done myself in about 20 minutes) he reminded me that he never promised they could remove the stain and this was the best they could do.

Turns out, it wasn't.

As a last resort, in the hopes of possibly avoiding a $1,200 paint job, I went online to research fiberglass pool stains to see if there were any other potential options (which is why I posted the images here and on a couple other forums). What I discovered was that this was an iron rust stain which, as I understand it, is among the most common types of pool stains, which can be entirely removed with ascorbic acid (vitamin C) which, as I understand it, is universally recognized amongst pro pool cleaners as the definitive solution. So after finding a pint bottle of ascorbic acid left by the previous owner (so is at least 13 years old) I added about a tablespoon to an 8 ounce glass of water then splashed it onto the side of the pool – then watched the stain completely disappear in a matter of seconds. I didn't even have to physically touch the side of the pool, let alone rub or scrub it with anything. I've since learned I didn't even have to do that. All I had to do was refill my pool and dump about a pint of this into it and let it run for about 24 hours.

So it seems it really comes down to this: The owner of this professional pool cleaning service, and/or the professional pool cleaner he sent out to clean it, were either aware of this simple inexpensive solution (that would save the pool from having to be painted), or they weren't. So this is either an act of fraud or, at the very least, gross incompetence, yes? Am I a victim of a scam, or just a pool cleaner who is really ignorant about how to clean pools? I really want to make sure I'm not out of line here before pursuing this further.

Thanks for any insights and guidance you guys can provide.


Pool Stain 1.jpg

Pool Stain After 1.jpg


Mod Squad
TFP Expert
Bronze Supporter
May 3, 2014
Laughlin, NV
Pool Size
Salt Water Generator
SWG Type
Pentair Intellichlor IC-40
Most likely ignorance. To be honest, iron staining is very unusual in our area.

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