You will have a bear of a time darkening the glass, you could darken the finish that you put on the glass with a little better help. Be careful using sand paper on old glass. You reduce the belly, or back a fraction you can develope a hing in the tiller that will cause some major problems. If it is starting to check, you can scrap it off with a blunt object like a dull bur scraper or the back of a knife. Just be careful when you remove the old finish. Dont want to have to retiller the whole thing and lose weight, or worse, case a hing or flat spot that will cause failure in the lams.