Peter, I was witness to many QE2 deckhands 'painting' the Aluminium during the let's call it dark hours, all they (seemed) to do was have a can of what appeared to be clear fluid (in the tin) ,roller it around the affected area , & hey presto it came out white, no prep involved, I might add the same proceedure was applied to steel handrails & etc, is that why all older ladies have layers & layers of 'clag'?. As a postcript to the transition joint problems , well poor old QE2 as well as ORIANA (the leader in these in retrospect problems) were constructed over years & years in the open, no such luxury as covered construction halls in them days , so, I guess condensation did have an affect. ( bet the shipwright got a friendly welder to warm the affected area before Araldite was applied)