Removing the impurities is a nearly impossible task, MoMo. Thats why I said it was a very tedious repair. I'm not harping over the darn thing, I'm just explaining (now with pictures from you) why welding on these cases is very difficult.
The repair is fine for your purposes, and in that area it will likely be fine also. But if that motor ever hits the pavement on that cover, you can fully expect that repair plus a larger area to be chunked out. Aluminum gets work hardened very quickly. To have filled that area required lots of heat time, even though the amount of heat appears to have been low. The only way to remove the impurities is to pre-heat, clean with Acetone or other degreaser, and repeat. The moment you start welding, the aluminum will purge more impurities that you were never able to get at. (You're welding at more than 1200* with aluminum).
The downside to Rod welding (if thats what was used) aluminum is it does not allow pre-heating of the surface with the arc before fill rod gets added. TIG is ideal for this. Arc, wait and watch, then when the surface is cleaned away from the repair, fill rod gets added. ITs very different than steel.
I think the world of the work you do, so please don't interpret any of this as criticism, only information for anyone that might read the thread and consider this type of repair. It can be done, but should be done with TIG with completely stripped cases so they can be properly prepared. That's all. And you'll still have to grind down the repair (as you're doing) if you want it imperceptible.
