Hi there
FWIW I remember this too - as distinct from anodising, which I also
know and have a had a go at. I was a schoolkid when I tried - about all
I can remember is:

- it must have been pretty string as I'm fairly sure it was a
satisfyingly 'bubbly' mixture, and yes I do know to add soda to water
- I think I first prepared the Al by scrubbing it with steel wool. IIRC
this was part of the process; the action of the Caustic solution on the
'grain' of the metal created this way was part of the point.
- I think the panels went in for only a short time, say 30secs.

Sorry I can't dredge up more. Have you tried somewhere like the
repairfaq?

jon N