HF 21 gal. air compressor?

I think so, yes. Does it have a cast iron compressor? Please do yourself a favor and don't buy one with an aluminum compressor. Even if it's just a small one. You'll be wasting your money. But as far as size, I think it will handle spraying fine. It'll run a DA too, you'll just have to take breaks to let it catch up. Also, what "I" would do is look on your local CL and maybe find a compressor with a bad motor or pump or just a tank. You can plumb another tank in with the HF compressor and probably do really well. On the cheap too. I've done it before.