Did you know that the giant mirrors of major telescopes regularly get a fresh coating of aluminum to keep them in top working order? It's not a simple process. Technicians have to take the back end of a telescope apart, remove the enormous mirror, clean it, put it into a realuminizing tank, then put everything back together. It's a little easier for a segmented mirror like Keck, because engineers can use the smaller spare segments to take the place of ones to be refreshed.
You can watch the process thanks to a time-lapse movie on YouTube. Richard Koolish forwarded me the link to a 3½-minute-long movie showing the realuminizing process for the famed 5-meter Hale Telescope atop Palomar Mountain. (One of the first things I learned after starting at S&T 20 years ago is that the telescopes aren't on "Mount Palomar.") I suppose that I found the movie especially interesting because I've stood under the massive telescope and watched as astronomers prepared for a night's observations.
The movie's uploader, "palomarskyguy," has two other videos to watch too.