Aluminium Welding - harder than it looks

I was quite pleased with my previous aluminium welding until I tapped it with a hammer and it broke. It turned out to be joined only in two very small places with a layer of dark oxide between the bead and the thicker square piece at the top. I had cleaned the paint off the curved part with sandpaper and the weld took to that part but the other I just degreased. I did take a small amount of advice from the internet but somehow missed that you really need to clean the oxide off.

I took new advice and this new attempt was much more successful, when I hit it with a hammer the base metal snapped off under the weld. I should have cleaned more of the paint off the bottom part, you can see the weld overhangs the break with a layer of dark oxide that prevented it from penetrating. There is also significant contamination on the surface of the weld which I'm going to blame on the paint too. The photo shows the weld after some cleaning with valve grinding paste and an electric toothbrush (this isn't every effective BTW).