An experienced welder could weld it without riveting first. For me by fabricating and riveting first then welding it separates the two process making it easier two keep all the parts in the correct positions during welding.
I have welded alum tanks before and while I admire your skills and attention to details, I dont think you did it correctly. The most disturbing part is the flange bending @90 degrees. Aluminum should never be bent in the non annealed state. It is too hard and will result in stress cracks resulting in fuel sweating on the walls of the tank I would even avoid bending the long sides. This is not a tool box drawer. All that flange business is a huge waste of time. You can butt weld all the sections, tack them and weld them. Regarding the baffles they can be fillet welded top and bottom first, no need for flanges. Lastly your holes for fuel circulations at the bottom are too small to allow the fuel to travel freely in a sharp turn when the tank is low. You need circular holes at the bottom of the baffle to avoid sucking air on a sharp banking motion. You want as much fuel as possible near the suction tube. I admire your craftmanship, but I can assure you we dont weld alum tanks like that. You also need to remove the oxide before welding with a chemical etching solution. I can hear you IF struggling to break the oxide layer. Way too loud. Alumine oxide is the reason you are getting the pin holes. Add a 1 percent helium in your argon, it will weld a lot better. Stay safe and busy. Regards from France.
Thanks for you feed back. I didn't design this tank it is a tried and true design with many preforming successfully in the field. 5052 with generous bend radius does not need annealing to prevent cracking. The baffle cutouts have a total area of ~1/2 inch, a half full tank it is capable of flowing over 7 gallons per minute through this size hole. While I am sure your techniques are sound I am not sure they are the easiest way for the average home hobbyist to produce a usable product.
