I hate the metalized coatings they put on lenses these days because they are so fragile. The antireflective coatings "craze" when exposed to thermal cycles. My current pair I ordered without AR coating, and they still came with a "hardness" coating. All it takes to ruin it is a spattering of aerosol sunscreen in passing.
A few weeks ago, one lens got bad enough that I soaked it in glass etching acid overnight and then polished it with Novus acrylic polish. In the last month I spent a lot of time outdoors with folk using sunscreen, and the other lens has gotten bad enough that I need to polish it as well.
Hah, it's great to hear that the etch-and-polish approach actually works! I've considered doing this to my glasses to remove messed-up antireflective coatings, but always chickened out before actually doing it.
You aren't going to get it perfect. There's always going to be fine scratches, and probably visible ripple texture when you look at the lens from several inches away. When wearing the glasses, though, it looks just fine. It's definitely an improvement from the damaged coatings.
On a previous pair with AR coating, I tried sanding rather than etching. It was a lot more effort, and I caused some distortion on the outer edges of the lenses. It was still an overall improvement from the messed up AR coating. I was taking high dose corticosteroids at the time, and the AR coating was driving me so crazy to the point that, if sanding didn't work, they were going to be smashed with a hammer!
That's the reason I go cheaper on sunglasses. You can find quite good ones for 15-20 and I go through 2-3 pairs a year. No matter how well one takes care of them the coatings and lenses get ruined quite quickly..
The one recommendation about going with cheap sunglasses is getting polarized ones. Good polarization is practically half the difference between the ultra cheap and the really fancy sunglasses in my experience. Decent polarization really cuts down on glare. There are several brands out there making pretty affordable pairs of glasses with polarization filters on them. I get mine for ~$35, often last me at least a year or so.
A few weeks ago, one lens got bad enough that I soaked it in glass etching acid overnight and then polished it with Novus acrylic polish. In the last month I spent a lot of time outdoors with folk using sunscreen, and the other lens has gotten bad enough that I need to polish it as well.