Do you have wood mulch near the windows? Are the spots clear or darker colored?

Yes on the wood mulch. The windows go down close to the ground and there is mulch right there. I would say that the stains are clear. They look like water marks.

I was thinking artillery shotgun fungus from the mulch, but that would cause dark round spots.

Sorry if this sounds silly, but if a razor blade will take it off, why not just use that? You can get a nice little handle that holds a scraper. Maybe it is too large an area to do comfortably with a razor blade?

After trying everything "safe" to remove build up on a granite counter top, I realized nothing was as effective as a razor blade and I wish I'd saved the money spent on products and just started with that.

Anything with "artillary" and "shotgun" in the name does not sound good. However, I think you are on to something because this is only a problem along the 5 windows and one slider in back that are adjacent to mulch. the other windows in the house have no such problem.

Since you got me thinking that it was some sort of algae I tried the anti mold stuff that I use in my shower and I think that worked the best of everything I tried. I will know better in the morning when the low sun shines through those windows. Hard to see it except in those conditions.

I could use a razor as a scraper but like you said it is a lot of area plus it is very subtle. I think the razor would probably leave some residue - it would be hard to do a thorough job.

I will report back in the morning on my results with the anti-mold stuff.

I dont think that the anti mold stuff made much of a difference. I think elbow grease is the best solution. Or dont look too closely during the 30 minute period in the morning when the sun is low and shines directly through those windows. I have chosen the latter strategy.

Haha, yes, not looking closely is sometimes the best solution. Just refreshing the mulch to keep the spores from spreading helps. It's a tough job cleaning it off completely though.

Did you ever try the CLR or limeaway?

No, but I did try vinegar which seems to work on mineral deposits. Didnt work.

My experience with vinegar for mineral deposits (and mind you this is just my experience), is that it works as well or better than the commercial products, but takes longer- usually a soak, whereas the commercial limescale removers sometimes take a few applications but work pretty quickly. I have an exterior window that has some build up on it. Maybe I'll go try some straight 5% white vinegar on it for a few minutes, and some LimeAway on another section- and I'll let you know the results. Not saying that this will equate to yours, but it'll show if vinegar is as effective... maybe... :^)

Well, here's an update... my deposits scrape off with a razor blade too, and like you were concerned, it's hard to get it all. That's to say that it's some kind of deposit and not etched.

The 5% vinegar for a brief few minutes worked as well, if not better, than the name brand limeaway. The deposits in both areas are lighter, but you can still see them. This is a window that I've procrastinated/neglected cleaning because it's behind a shrub and I wasn't sure what to use. I'm going to keep thinking about this from the standpoint of my neglected window and I'll let you know if I find anything that works adequately, though I'm afraid it's all going to involve a lot of elbow grease. Sorry to not be more help.

Thanks for the feed back. I gave up and called it "good enough." Good luck to you.