FYI, I wouldn't sandblast before welding, especially if you're using silica sand. It will embed silicon into the surface causing a poor weld. Aluminium welding can be tricky and much more temperamental than steel. Whoever welds it will do the final prep just before welding. It should be wire brushed with a stainless steel (not steel) brush no more that 20 minutes before welding, since aluminium oxidizes very fast (at least from a welding cleanliness perspective). Cleanliness is usually the primary problem if there are problems with an aluminium weld.
Keep in mind that when welding any tempered aluminium like 6061-T6 (a common extrusion alloy) the weld area will become annealed and only be 1/2 the strength, so avoid welding around higher stress areas or add gussets. That however, shouldn't be an issue with your repair.
Hope this helps