I've built a few trailers. Used plain rivets on the first one and screws on the second. On the third I bought most of my stuff from a truck hardware supply and used rivets that take a "button" afterward. These are awesome and don't damage the panels because they're also aluminum (screws corrode the panels due to electrolysis of dissimilar metals). I always painted the entire frame before installing the panels both to prevent rust and so the bare aluminum wasn't contacting the bare steel. If you go with screws or rivets, don't forget to apply sealant to every seam before installing the panels. Panels will warp from the heat of sunlight so at the seams you want rivets at least every 6 inches. You want panels that are 49" wide to give you the necessary overlap.