I did the same thing with my SAR-1, and other than periodic cleaning and oiling, have done nothing else. Must be something about the type of metal used.
Now, go shooting in the rain and you'll see plenty of rust spots show up everywhere else! But as long as you clean and oil right afterwards, no harm, no foul.
Got to love the AK's - couple of weekends ago I attended a two-day carbine course set up by Cross-X in Worcester. Ran over 1200 rounds without a stoppage.
If only the AR guys could say the same thing.... (he,he,he).