My Kohler tub with the textured bottom is 17 years old, but in the paperwork which came with it for rust removal they recommend Bon Ami or Super Iron Out Rust Stain Remover. I suppose Barkeepers F could work as well, and I have used it.

Either you have naturally acidic water or your water softener needs adjustment.

Pure water has a pH of 7. If the pH level goes below 6.5, water is considered acidic, or “soft”. What a water softener does is increase the acidity of the water. If it is too acidic, however, it begins to corrode the copper in your water lines. As you surmised, it is the copper oxide that is causing the blue stain.

If you have a water softener installed, ask your softener technician to adjust the PH so it is at or just under 6. That should stop or at least reduce the problem.

If your water is naturally acidic then the solution is more complicated generally involving the installation of a chemical feed pump to inject a solution of soda ash into the water to raise its PH.

To clean the stain, never using a scouring powder. Bon Ami was recommended by Kohler years ago because it was the only cleanser then commonly available in supermarkets to use feldspar and limestone (collectively called "rottenstone"), considered "soft" scouring particles rather than pumice, a very hard particle.

Still, it is a scouring powder and will micro-scratch the tub's surfaces. Micro-scratches harbor dirt and micro-organisms, making the tub harder to clean next time, and the more your scour it, the harder it is to get clean next time - a vicious circle.

Barkeeper's Friend (BKF) cleanser is a chemical scourer rather than a mechanical scourer and is safe for most enameled cast iron. BUT, always test in a small, less noticeable spot on the tube before using. Also be sure you buy the cleanser. BKF makes a specialized cleanser and polish especially for metal cookware that does contain soft scouring particles and should not be used on enameled fixtures.

Your copper piping should not be the problem. Porcelain over cast iron Kohler tubs clean well with a light application of Bon Ami and gentle cleaning, or 20 Mule Team Borax sprinkled on a wet sponge, lightly applied to surface, then rinsed well. I'd agree that the hardness of your well water may be to blame. Don't use bleach or acidic based cleaners, as it can invalidate your warranty.

@StarCraft Custom Builders - when you suggest BKF cleanser, are you referring to their powder, or is there a specific cleanser that is appropriate for this application. Thanks for your input, I'm "chemistry savy", I was able to do some research on water purification systems, etc.

kohler makes an enamel sink cleaner. it's on amazon. maybe you could use that.

@ Laura Grosmaire

We use the original BKF cleanser in powder form. Use a sponge or soft cloth (better than a sponge), not scouring pad.

We have very acidic well water and copper pipes which gives us blue water stains on sinks and tubs as well. I have always used CLR which washes it away very quickly. I have never seen anything hurt by it in years of usage. I also use Barkeeper's Friend (powder) on porcelain as well for a general cleaning.

Thanks jzecnelson! Does CLR = chlorox?

No, it isn't Clorox. I should have been more specific, sorry. CLR is a household cleaning product. CLR stands for Calcium, Lime and Rust.

I live in NC and there is an apparent big need for it here as it is sold everywhere. I've seen in in Lowes, Home Depot and Walmart. Even the Dollar Stores carry it. The main ingredients are: water, lactic acid and gluconic acid.

Be a bit careful with it around faucets. I've had it take off some of the plating on the edges of some faucets while others seem immune to it. Here is a link to their site: https://clrbrands.com/?&utm_campaign=9513700973&utm_source=google&utm_medium=cpc&utm_content=421637976893&utm_term=clr&adgroupid=97443641699&gclid=CjwKCAiArbv_BRA8EiwAYGs23HxFVuOLZNfP3Beh_JEtHt-28B3EAEiZuUm5jzHj0h8jrawXgYZ8-RoCdGAQAvD_BwE&gclsrc=aw.ds

Good luck!