aluminum dulls when tig welded

by Josh
(Boise, ID)

I was wondering why aluminum turns dull when I tig weld it, but other times it turns out shiny? I don't do anything different. Is it the type of aluminum? I prep all aluminum the same way, i sand it really well wherever I weld it. I don't use a wire wheel because it seems to contaminate it. I use 2% thoriated tungsten and R4043 rod.
Thanks

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In my opinion , it has to do with cooling rate, travel speed, and size of argon envelope. the cleaning action will only go where there is argon shielding and that makes it look frosty sometimes, while other times when the argon shield is smaller and less arc energy is used from the metal being hot, the look is different.