Yea, what Greg said!! I have taken 30 years messing about with my B Rapide engine, now I am catching up, as previously stated, my life is running out!
Alodine & Alochrom are both trade names for chromic acid solutions applied to aluminium before paint. Trev was on the right track about timing, we were taught 24 hours rather than 30 minutes between alodining & painting.
The process we used was, for used parts, trich vapour bath (yes, another great thing become extinct) then left to cool, followed by scrubbing components clean in a soap solution, also used for new components, until we had a "water break free surface" which means that when you run clean water over them, there is no breakaway between the water & the surface. Break away indicates oil on the surface. That was followed by immersion in chromic acid solution for 5-10 minutes until the item discoloured to a straw colour. Then wash & dry & make it the painter's problem.
If you (can) buy chromic acid powder for making your own solution, wear dust mask breathing protection, gloves, disposable overalls & wash extremely well. Chromic acid dust in your lungs is extremely dangerous!
Better still, do what Trev does, make it someone else's problem!