I've saturated it in white vinegar as well as CLR with no success. Thanks.
Very carefully try muriatic acid. It's used to clean mortar from bricks. But dilute it well, stucco is very close to the same thing as mortar.
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Just read my Diggers - Hydrochloric Acid ( muriatic ), 1 part acid to 10 parts water is used for brickwork and mortar. Try it even weaker if you wish, and always start in a small unseen area and leave for 2 hours to be extra sure of 'wrong damage'.Use gloves, glasses and be outdoors. It corrodes skin ( kind of itchy at first, good sign to wash hands ) and it also destroys anything iron and other metals that may be near the acid surroundings. ( For example, my laundry sink and tap fixtures ). It also has a very distinct 'fog' that comes out the cap when you take it off. Very dangerous - don't sniff. If what you have is small, mix a solution in a plastic bucket and leave the object submersed until it is clean ( keep a close eye on it, it could start to eat further than the stain ), remove with tongs or something and wash off with water.If it is large ( like tiles ), mix a solution in a weed killer pump ( the ones that you can hold on your back ) and spray the area with solution, and then wash with water or mop until it stops foaming as the foam is what cleans deep.When it is dilute it is pretty safe, it even has medicinal purposes. But concentrated straight from the bottle, its very toxic, has very potent fumes that shouldn't be inhaled at all. Sorry for over - info on safety, but I felt it couldn't be suggested without proper precaution.
You should re-post this as a separate answer. It deserves the best answer badge.
I was more just continuing your answer with some safety notes. I'm not searching for a badger, but I think the correct answer is below by Burf
Wow! Thats a lot of information. Thank you so much. It sounds like it's going to be a task to get it off and keep it off. Unfortunately, the initial stains come from the sprinkler system and while I can adjust the sprinklers to miss the house, we are in a valley, so we get a lot of wind. Wind that would just blow the water right back onto the house. :/ Now, this may sound like a silly question, but my husband and I were both truck drivers and we hauled hazmat on a regular basis which means lengthy background checks. Should I expect any problems if I try and purchase acid? Thanks again.
You can get the acid at some building supply stores or swimming pool suppliers. There should be no paperwork to purchase it.
The way recommended in the UK is to let it effloresces, but NOT to wash it otf, that only causes it to be driven back into the wall. Vigorous brushing when dry makes the salts fall away from the surface, where they can't form unsightly stains.
I've built quite a few stuccoed structures and have dealt with a number of experts regarding efflorescence (calcium carbonate or calcium sulfate.) The first thing to consider is that calcium stains emanate from within the stucco itself and spreads to the surface through the microscopic pores in the stucco. You can clean and remove the external staining but it will likely return. There is only one sure way to prevent the recurrence of the staining and that is to eliminate water from getting to the stucco. Without water, efflorescence cannot occur. Make sure that the soil around the building is lower than the bottom edge of the stucco and that there are no penetrations in the stucco where water can enter; such as around electrical fixtures and plumbing lines, cracks in the stucco, etc. Caulk those penetrations with a high grade, waterproof caulking.Once you have taken care of all the sources of water entry, you can work on removing as much of the staining as possible. Scrub as much as you can of the dry efflorescence off with a stiff bristle brush or broom. Then prepare a solution of 5 parts muratic acid to 100 parts water. Saturate the stucco thoroughly with water and then brush on the acid solution. After you have removed as much of the efflorescence as possible, neutralize the area with a dilute solution of household ammonia and rinse with clean water. Its not perfect, but short of painting the stucco with an elastomeric paint, its about the best you can do.