Re: condensation ring on window
I've seen similar patterns where the installer has used their suction cup "handles" to hold onto the window while installing it. It leaves behind oils from the rubber. Also, as a previous poster mentioned, there can still be adhesive residue left behind from the manufacturer's sticker, even though it's not obvious and you've cleaned the glass.
First, try to find a product called De-Solv-It. It's the best citrus solvent I've found, and it does an excellent job of removing oils and adhesives where other products fail. It will leave it's own oily film behind, but you can easily clean that off with some other cleaning product. It's kind of expensive and hard to find: around $8 for a 12 oz. bottle. WalMart used to carry it, but the last place I found it was the local Ace Hardware store.
Try some vinegar. Sometimes a little bit of acid helps to break down stubborn residues, especially if it's minerals bonded to the glass.
If that doesn't work, try a mild abrasive designed for removing water spots off of glass. You can usually find this at auto parts stores.
The "Senior Member" designation under my name doesn't mean I know a lot, it just means I talk a lot.I've been a DIYer since I was 12 (thanks, Dad!). I have read several books on various home improvement topics. I do not have any current code books I can refer to. I was an apprentice plumber for two years.