Aluminum is a bear to weld and requires special equipment for the most part, MIG works very well.
Pop rivets are a quick way to join pieces of aluminum flashing.

A local Propane Supplier has rods that are used to weld aluminum with an oxy/acetylene torch. Although I have wanted to try it, the occasion has not arisen for me to do so as yet.
The rods cost about $1.00 each here.
Good Luck,
Junkmanme

i think what junk was saying is true most welding supply outlets will have the Alladin 3 in 1 aluminum brazing rods. You will need a new SS hand wire brush a few of these rods and a oxy acy brazing rig. The welds would not be considered structural such as for handrailing but do a nice job of seal;ing or joining fairly heavy (not paper thin) pieces of sheet aluminum together. Note a turned edge is the preferred method of joining since the aluminum sheeting tends to warp from the heat that is required to make the joint braze. If you have access to a sheet metal brake you can form it up anyway you like.