Alkali Staining and Sealing Limestone WaterLine Tiles

[MCF50]

So here is a question - how do you seal your waterline tile? I made the mistake of using white limestone for both my waterline tile and my coping. The issue with this is the coping will get brown alkali stains on it every couple of months and the only way to clean it is with muriatic acid - yes I know not good for the limestone, but it is the ONLY solution I have come up with to remove the brown stains. Tiles when from brown to white again with a diluted mixture. The stains come from moisture getting into the limestone from BELOW, making it's way to the top, evaporating, and leaving brown residue on the top of the stone. We are BONE dry here in Houston so I am thinking moisture may be working it's way thru the white limestone water line tile and getting into the ground under the coping. Two concerns - 1. is it ok if the sealer gets into the pool water? 2. If I lower the water to below the waterline tile and seal and the sealer drips down the plaster it will leave 'streak' marks as that part of the plaster will be sealed not letting it get wet (we had this issue during build and they had to use some pretty nasty chemical to get the sealer off the plaster). Thanks!!!!

Any other ideas welcome - I don't mind using diluted muriatic acid to clean the limestone on occasion (not good for the stone), but that is just a band aid. Need to address where moisture is getting to under the coping and the only thing I can think of is the waterline tile being wet 100% of time of leaching some moisture into the ground under the coping. On a side note, the coping tiles above the strainers don't have this issue (because there is no ground beneath them). Thanks!!

 

[Toxophilite]

White limestone, Austin stone as we call most of it here, will almost always yellow a bit when wet and turn pure white under sun drying. Then, you have issue of areas where moisture contributes to the leaching and oxidation of minerals within the limestone itself. And, you can even develop molding where the moisture can't escape or stays wetted. As you note, it's a problematic stone in some uses. I have exterior wall elevations and mailbox of it, and powerwashing cleans it up. I don't seal it because I need to get to the staining, and sealer would just trap it under.

 

[MCF50]

White limestone, Austin stone as we call most of it here, will almost always yellow a bit when wet and turn pure white under sun drying. Then, you have issue of areas where moisture contributes to the leaching and oxidation of minerals within the limestone itself. And, you can even develop molding where the moisture can't escape or stays wetted. As you note, it's a problematic stone in some uses. I have exterior wall elevations and mailbox of it, and powerwashing cleans it up. I don't seal it because I need to get to the staining, and sealer would just trap it under.

Thanks. This is not yellowing. It is 'browning' of the stone where water is pulled up to the surface and then evaporates. "Alkali stain is caused by alkali-containing moisture moving through the limestone from the back side or bottom edge, depositing minerals on the face of the stone as the water evaporates. It can also be caused by ground moisture absorbed through the face or edge of the stone". You can literally see the outline of the gunite under the coping with the shape of the brown stains. Yes, they did put a moisture barrier between the coping and the gunite. I am thinking (not sure) it may be moisture 'seeping' thru the white limestone waterline tile. NOTHING and I MEAN NOTHING was able to remove the brown discoloration on the coping. I tried every 'neutral' limestone cleaner available, bleach, and even heavy power washing. Nothing touched the brown staining. One day I was so irritated I just took a little muriatic acid and put it on an area and it was gone! So I used a sprayer with water/muriatic acid and sprayed all the limestone coping tiles and they all turned white again. Night and day difference. Yes, I little etching due to the acid but I could care less about that issue as it's outdoor around a pool. I thought I solved the issue so I sealed the coping (not waterline tiles) with two coats of deep penetrating sealer. After 4-5 weeks the brown stains are starting to appear again on the coping. Perhaps 'acid wash' the coping again but this time don't seal it (I assume the 'acid wash' will remove any sealant). Thoughts? I'm open to any idea.....last resort is to replace the white limestone coping with something white that is non-porous.