I have had good luck with toothpaste for ones not too far gone and for ones that are worse, use 1000, then 1500 then 2000 grit sandpaper, THEN use toothpaste or semichrome. Take your time, first they will look worse when you sand them, wet sand them. THen by the time you get to the 2000 they will look good, toothpaste final polish will make them look very good. It takes a while but they can be brought back. Or buy new lenses.

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Christopher Henkel
1990 190E 2.6 - Arctic white SOLD
1986 190E-16v - Blauswartze
1993 300CE - SOLD
2003 W208 CLK 320 Cabriolet - Magma Red