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Loose rivets need to be replaced. Cracks should be repaired by riveting an aluminum patch over the crack with a good sealant (such as 3M 5200) between the patch and the crack.
Lots of good info in here:http://www.faa.gov/regulations_policies/handbooks_manuals/aircraft/amt_airframe_handbook/media/ama_Ch04.pdfSee the section on Stop-drill cracks and on doublers.5200 works fine on aluminum, I've used it many times.
do you know a welder? you can rent an aliminum welder, that would probably be the best way
you can't weld it. you need to install new rivets as stated. You got good advice from CP.The only thing CP didn't catch was to goop up the new rivets before you install them, probably with that same 3m stuff he mentioned. If you lived close I'd help you buck in new rivets, it's a two person job and best left to someone who knows how to run a rivet gun.