I was going Steel (again, I am rebuilding and the worst part was the skin) and then I thought better of it and going Aluminum on mine to replace the rusty steel that was on it. I can buy a prefinished sheet of .030 for $$60 or so a 4x8. No painting and light. Pop riveting it on with 9/32 rivets.
this is the place I am buying from. They also have tread Brite.
www.wrisco.com
DO NOT use Silicon (Bathtub caulk) for a sealant. It sucks. It will come loose and then wick the water in and hold it and cause corrosion. Then it leaves a residue that you can't get anything to stick to till you grind it off. Use a product like Vulkem or Sikiflex. That stuff will become structural and never let loose. These are products used for assembling campers including Airstreams.
http://www.vintagetrailersupply.com/category_s/24.htm