Probably known, but new to me.
Had some '68 front side marker lights I am restoring. The old lenses were white-ish and grubby. Soaked them in a strong caustic soda/lye solution overnight and they came out looking almost new.... no buffing to wear down letters, and a very good result, to me....
Going to try it on a pair of Cal Special taillamp lenses that are pretty grubby, with the "waffle" pattern on the backside almost impossible to clean with mechanical methods...