The problem is that when you get an aluminum alloy nice clean and looking like new, you have removed a thin layer of oxidized aluminum .
The one thing that alloyed aluminum likes to do, is to oxidize to that gray look .
Only way to prevent this, is to apply some sort of coating to the metal surface, to prevent oxygen combining with the aluminum .
I've been around aluminum skin aircraft for the last 40 years and I've watched this repeatedly the last 4 decades .
You have an unending cycle of cleaning and oxidizing, unless you get a coating on the bare clean metal, you will chase this endlessly .