Details on using Phosphoric Acid for Hardwater Stains?


I had a couple questions on using phosphoric acid to remove hardwater stains. My wife and I run a residential green cleaning business, and one of our clients is a marijuana farm, and they’ve got a North-facing, dark tinted window with some horrible hardwater build-up on it. I’ve tried diluted white vinegar, Bon Ami, Ettore Water Spot Remover, 0000 Steel Wool, Bronze Wool, and then Barkeeper’s Friend, which I hate using, (I have asthma and multiple chemical sensitivities and have to use a respirator so the Oxalic Acid dust doesn’t make my lungs burn), and nothing even made a dent. The windows would look clear when wet, but as soon as you would wipe or squeegee the water off the stains would immediately show. It’s not a huge priority for them to get it clean, (it’s a pretty mellow environment there, haha), so I have some time to experiment. There is a number of newly planted trees and vegetables a few feet away from the window, so I’m really hoping to find something biodegradable with a known low-risk profile.

I’ve read about some of the acids used, and received suggestions in my last post on the subject, and phosphoric seems an effective solution, (thanks for the tip, Steve076) but there is scant solid, tried and true information on it that I can find. It’s biodegradable, and seems safe, at least after diluted. I’m not interested in using hydrochloric or hydrofluoric, or for random cocktail of potentially toxic, recently invented and synthetic chemicals (Propylene Glycol, lets say, running off the sills into the soil and plants is not really in line with my principles).

  1. What is the effective range of phosphoric acid dilution? I can find 10% and 85% solutions online.

  2. What is the range of safe concentration levels?

  3. Are there any special precautions, or surfaces which might be marred or etched by it?

  4. Is it safe on tempered glass?

  5. Is it necessary to seal the glass afterwards? I have seen many mention that whenever acid is used, a sealant needs to be used. What are popular options around here for products? Any non-toxic, biodegradable options? (Does this advice even pertain for something like vinegar, which seems to work on certain hardwater stains)?

Any help on any of these questions would be greatly appreciated!


Get the highest dilution available, once you dilute it it has a 8 hour shelf life.

if using indoors I would wear a respirator , I’m really not sure I’ve split straight 70% on my hands and still have skin on them lol

Only really when using it around plastic type fittings, it will not etch glass unless you have it in a 200C oven, it will discolor plastics it will turn black to a grey and can stain colored pavers/concrete


Since unlike hydrochloric and hydorfluric which do etch the glass on a microscopic level, the pours of the glass are left intact with phosphoric acid sealing the glass is not necessary but if the problem is not removed it would be best to for easier cleaning in the future.
I’m no chemical expert, I have used it in the past and done my own research, someone else might have something other to add.


Excellent info, thanks!


You’d be better off just resolving the problem using the chemicals you don’t want to use based on your beliefs.

You’ll do more damage driving your vehicle to that job site then you will using the harsh chemical on the one window.


Or get a polisher and some cerium oxide and just be done with it.


Good tip Trenchfeet, I’ll look into it. Although if it functions as an abrasive wouldn’t it be liable to scratch tempered glass with Fabricating Debris?

jhans, I’ve been cleaning houses and hospitals for years. Sometimes the harshest chemicals aren’t even the most effective, and it’s sometimes just an issue of laziness to assume that the harshest, nastiest industrial chemicals are going to be the best. I’ll destroy any toilet stains and bowl ring in the world with a Kitchen Stone in about 5 minutes, leaving pure, white, unscratched porcelain. If I would have just stuck to using bleach, I would have been poisoning myself with carcinogenic dioxins and chlorine fumes for years (When I worked at a hospital I actually got a chemical burn in my trachea from bleach fumes. That is not fun). That toxic shit adds up in your bodies, and alot of toxins adds up in the environment. There’s a reason our oceans are dying and the planet has become a toxic waste dump. Ideally I’m making large strides towards living a sustainable, low-impact life on a permaculture farm, and I’m already growing a majority of my own food. So driving to a job site is a necessary evil for the time being, and I’m not going to make perfection an enemy of the good.


In a controlled fashion. You scratch it as deep as you need, then with a DIFFERENT pad, polish the scratches out.