I am using the following steps for treating my aluminum parts:

- scrub clean with Acetone and scotchbrite
- dry
- scrub clean in Metalprep-79 and scotchbrite
- rinse in water
- repeat until water forms smooth sheet (water break test)
- dip in alodine 1201 until part turns golden brown
- rise in water

After the final rinse, the golden brown coating feels soft and
delicate, and could be rubbed off easily. I have to hang the part and
let it completely dry before touching it.

Now, I can remove the alodine by scrubbing the part again with
Metalprep-79 and recover its original shiny surface. If it repeat the
above surface treatment all over again, I get a more stable coating
that looks more uniform and not so soft, and also appears slightly
darker in color.

I am not sure if I understand why this is happening. This has happened
every time I tried it, not just once, so it is not a random occurence.
Why is the re-coat giving me better results than the first-time coat?