Am buyingf a house (Sacto CA). Instead of a back yard, it has an old pool/spa. This may or may not explain the prescence of the crud on the sliding door.
There are 2 sliders (family room, master bedroom) - only one side of the family room door is affected.
This looks like shower door crud, but:
Magic erasers don't work. Bathroom cleaners don't work. Straigfht phosphoric acid (sold to prep aluminum for coatings) removed most, but the outline of the streaks remain.
Before I take a hammer to it, any other ideas?
Yes, I know about citric acid and vinegar and CLR and sulphuric acid. Each will be tried in turn.
But: WHY is this so tough? How does this form on an exterior pane to begin with? Is there something about the proximty to the pool which could explain it?