Though you may not actually see it, Aluminium will oxidise with a thin coating within seconds after cleaning. That oxide actually protects the metal underneath and is not good at receiving paint. As an aside, when I worked in the electricity supply industry, in order to solder onto aluminium cable sheaths, a stick of abrasive solder needed to be continually rubbed onto the Ali, whilst applying heat, to cut through this oxide coating, whereupon a strong joint could be achieved. Similarly with painting aluminium, you need to break through that oxide coating before paint is applied. The only reliable way is to apply an etching primer and let it dry for a few hours before building up several paint coats on top.
