What rivet sizes does this handle?

The mandrel set covers standard sizes used in automotive sheet metal and panel work. Check your rivet specs before ordering to confirm diameter match.

Do I need any other tools or supplies to use this?

You'll need rivets themselves (not included) and a firm work surface. A backing bar or bucking block on the back side of your panel makes the job cleaner and faster.

Can this gun handle aluminum rivets, or is it steel only?

Manual rivet guns work with both aluminum and steel rivets — material compatibility depends on the rivet, not the tool. Just match the mandrel size to your rivet diameter.