For what it is worth, I've got these tips.
For the gunk adhering still to the aluminum/steel surfaces, try water on a test patch. Hide glue was also used to glue coverings onto metal, in which case the patch might just come loose with a slightly damp sponge. If that doesn't work, then it probably was shellac, and you can use other solvents in the same gentle manner--you won't hurt the metal. I've also popped stubborn bits off with jewelers' screwdrivers.
For white spots on aluminum, these are usually aluminum oxide, a very hard substance. Use a fine grade of corundum sandcloth to burnish these spots down to the metal, and you are done. You can also sand off rust, but if you use rust remover, make sure it is very thoroughly cleaned off at the end.
"Green gunk" around screw and rivet fittings is another form of reaction between dissimilar metals. Use a split wood matchstick or toothpick to scrape the stuff away, wipe clean with a cloth, and put a thin layer of clear nail polish over the area; this helps impede further oxidation around the contact.
I use hide glue wherever it will adhere to metal, as it is water soluble and thus re-doable. If hide glue won't adhere, water soluble glue stick adhesive (acid free from scrapbooking shops) is another good, reversible choice.
I hope this is helpful; weigh it among other expert's advice as well.