Corny keg as air tank (somewhat OT?)

Hey homebrewers - I have a small air compressor with a rather small tank (1.5 gal), but it mostly fits my needs for light duty. Though, every now and then I encounter a slightly more demanding job and wish I had a larger tank so that the compressor isn't cutting in so often. Given that I often have an empty corny keg I begin to wonder if its safe to pump it up to ~120psi to use as a portable tank? The compressor's manual states that the air from the compressor "is not safe for breathing", but I'm thinking it would be fine in the keg. A dedicated portable tank is fairly inexpensive, but it takes up valuable space.

And yes, I realize that I really shouldn't have an empty keg sitting around, but that's life.

Thanks,
Isaac

Corny kegs are typically rated to 120-130psi, but you might want to consider an appropriate de-rating because of age.

How do you plan to regulate the output pressure from the keg? I know none of my air tools take 120psi directly.

I agree. I think the PRV on the corny is usually set to 60 psi? Is that right? At any rate I wouldn't push it past that. And I'd be hesitant for using the keg for beer again, even if you have an oilless compressor all kinds of crappy bacteria can live in the condensate.

Thanks, guys. I have a spare regulator and necessary connectors, but that bacteria comment has scared me away from the idea. Though I do have a boatload of StarSan.

Isaac