I would go the way of the razor blade. It really is hard to scratch glass with this process once you practice. Practice on an old piece of glass first.
Gotta try that one ^. If you have water mineral buildup on the outside with no fish I've found auto compound does wonders. Just have to wash the heck out of it once you're done.
Etching can look like mineral build up. I'd say if the razor doesn't get it then you likely have etching. I don't think you can do much about that except to keep the tank full.
what i'll do is dillute the vineger in a bucket of water then squeeze the sponge out and use some paper towel to catch any drip... will be minimal and i'm going to be changing the water several more times before the thing is ready for fish ...
the whole point of using vinegar is that the pH of the vinegar is strong enough to dissolve the calcium buildup (water line deposits). if you dilute it in water, it won't work anymore.