When I bought my SL the airfilter cover was frost white basicly. I started out with a wire brush and worked my way down to using #0000 steelwool with some aluminum polish by Bluemagic (small blue can/jar) It really made a difference. I can't say it got to where it was from the factory, but it definetly shined it up.

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1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
MBCA member