White marks on fibreglass - efflorescence? Have pics...

[AU Pete]

We have a fibreglass SWG pool with travertine tiles around it.

Recently we had a lot of rain and the pool filled right up to the tiles for several days before i had a chance to backwash.

A few days later i noticed 3 or 4 'drip' spots directly below the grout joins of the tiles - where there were dried white marks on the fibreglass above the water line.

i scrubbed them with the pool brush and a rag, and i’d say it’s half gone - yet some have remained/come back. (they 'disappear' when you splash water on them - but you can feel the rough texture with your finger remains)

I’m guessing this is most likely grout efflorescence right?

Confirming - the marks only appeared after the recent heavy rain and water level reaching right up to the tile level.

water readings are currently:

FC 5
CYA 70
PH 7.6
TA 50
Salt 3800
CH 225

SWG running at 20% for 3hr40min a day (its only spring time here in Australia)

water is clear and has been for about 10 months.

I have a few questions...

In terms of cleaning them off:
I’m hesitant to use diluted Muriatic Acid to attempt cleaning these marks for 2 reasons, A) I don’t want to damage the gel coat B) I’m nervous about handling small quantities of MA, and splashing it around trying to get a 1:4 solution with water. Dripping MA on the tiles makes a fizzing sound and scares me, letalone what it must do to the gelcoat. should i be this concerned? any tips for handling?

As a less brutal alternative - I gather plain vinegar used undiluted might be a less severe way to try removing these marks? Is that safe/safer to spray onto the marks and try removing them? i’m thinking I could try spraying and leave for a progressively longer amount of time to see the least amount that will get the marks removed.

In terms of stopping the marks coming back:
The travertine tiles were sealed initially with this product - but i may not have got the sealant right in up there to the underside. Will extra coats of that product stop this problem reoccuring - or is there any way to stop it? I was thinking perhaps some sort of resin/glue could be used to really seal the underside of the grout and stop the water splashing up in under there. Any tips/ideas?

Heres some pics. Any help appreciated - Pete

 

[Texas Splash]

If the residue is from an efflorescence effect, I would assume it's from the tile/stone and not the shell as that's the only place I could imagine moisture penetrating from the outside inward. You should not have to worry about using reasonable amount of acid on the gelcoat as long as you apply it and rinse immediately afterwards. I've poured 100% strength acid on my shell steps with no ill effect (it was a manufacture's test). Since you are hesitant to handle the acid, maybe consider using an application tool such as this one: Amazon.com : Purity Pool OS Out Spot Stain Remover : Swimming Pool Stain Removers : Garden Outdoor. You could also try a test area with a Magic Eraser or something like a 1,000- 2,000 grit wet/dry sandpaper. Always try a small inconspicuous area first to ensure then end (dry) result meets your expectations.

As for why it happened, I can only think all that moisture made its way from behind the stone and dripped down onto the shell. Maybe others will have a thought or two.

 

[AU Pete]

I can only think all that moisture made its way from behind the stone and dripped down onto the shell. Maybe others will have a thought or two.

Thanks for the reply Texas Splash.

I'm happy to report I was able to remove the marks with the first/gentlest attempt. I used a toothbrush to rub some vinegar on the marks. I found that after leaving on for a minute I could then gently rub the smooth rounded plastic edge of the toothbrush head on the marks and they crumbled away. A few minutes of repeated rubbing seems to have removed them all and they haven't returned so I think it's all good. I'll be sure to keep the water level down from the top edge in future.

 

[Texas Splash]

I'm happy to report I was able to remove the marks with the first/gentlest attempt.

That's good to know. Always like a happy ending. Enjoy your swimming season.