Why is the inside of my ALUMINUM rim rusting??

[devries]

I have a 1990 cr500.
The metal on the inside of my rear rim is starting to flake? Is there a different metal compound on the inside of the rim that has a tendancy to rust and or flake? the rim is still solid but i want to take some preventative measures so it doesn't wear all of the way through. Any suggestions?

 

[Pantaz]

Probably lacquer

Some aluminum products are lacquered to prevent oxidation. Over time, especially on something that sees a lot of grit, like a dirt bike rim, the lacquer can fade and/or flake away. I've often had this occur on street bikes with cast aluminum wheels.
Use aluminum polish to clean off the wheel and give it a good shine. I've used "Mother's Mag & Aluminum Polish" with excellent results.
http://www.mothers.com/
As long as you keep the wheels clean after riding, oxidation shouldn't be a problem.