We used rivets on sheet metal for the first time last year since it was our first sheet metal frame.
We also used rivets to attach our tread to our HDPE flywheel in our shooter this past year. We did got with stainless steel on this application though.
We used Clecos
http://www.mcmaster.com/#cleco-style-clamps/=pvbbwvto help hold the sheetnmetal in place while we were riveting. We also use these for attaching our bumpers onto the chassis during the tournaments.
We have several pairs of Clecos pliers
http://www.mcmaster.com/#cleco-style-clamps/=pvbcww