Can I use 120 grit on my fibreglass pool?

tonylewis

Bronze Supporter
Jan 16, 2018
4
Canberra
I have calcium scaling on my pool, and while the water level is low (separate issue) I want to get rid of the visible part above the waterline. I tried hydrochloric acid, but it didn't budge it, so I understand it's calcium silicate. Sandpaper seems to work, but I'm worried about damage.

I got some 240 grit pads for the detail sander, but one pad wore out very quickly. I worry that if I use a coarser one it will damage the pool.

My question: is 120 grit on a detail sander going to be OK to use on the walls of the pool?

Secondary question: if I do a test and it feels smooth, will that be enough to know that it won't do damage?
 

Texas Splash

Mod Squad
TFP Expert
LifeTime Supporter
Welcome to TFP! :wave: So, being new to TFP and not knowing much (yet) about your specific gelcoat situation, I'll address a couple things:
1 - How did you come to the conclusion it is calcium silicate? It would be good to know more about your water testing history as well. To my understanding from one of our experts, calcium silicate is not soluble, therefore it shouldn't dissolve in pool water and attach to the shell, but perhaps @JamesW can shed some more light on the silicate angle.
2 - 120 grit can be used, but it may leave some hairline scratches, so test carefully to see if you are satisfied with the results. Checkout MY STORY to see how your scenario compares to mine. Even with 120, it's hard work and lots of sanding, but it can help to cut through the chalkiness and bring the original color back. Any residual hairline marks may not be a big deal for you, but test first.

After you get a chance to provide some pool water history (include which test kit you are using as well), and read my story/thread, we'll see what else we can do to assist. Don't forget to update your signature too! Nice to have you with us. :swim:
 

tonylewis

Bronze Supporter
Jan 16, 2018
4
Canberra
@Texas Splash I tried applying hydrochloric acid to the stain. My understanding is that if it is calcium carbonate, then it should bubble and dissolve easily. It didn't budge.

@JamesW I'll take one as soon as possible. It's raining here for the next few days, so it might take a bit of time to dry out enough to show the problem.
 

Texas Splash

Mod Squad
TFP Expert
LifeTime Supporter
My understanding is that if it is calcium carbonate, then it should bubble and dissolve easily. It didn't budge.
That it is correct, but because it didn't dissolve of fizzle with acid doesn't mean it's calcium silicate either. It could simply be a defect in the gelcoat or that something happening chemically allowing the gelcoat to get that oxidized, chalky look - if that is indeed what you are experiencing (like the ones in my photos). We'll watch for some photos when your weather clears up. Have a good day.
 

tonylewis

Bronze Supporter
Jan 16, 2018
4
Canberra
The rain has foiled my plans, and the water level is too high to take photos or do anything about it. I'll delay plans until the next time there is a lower water level. Thank you for the responses.