Halakhic Positions of Rav Soloveitchik, by R’ Aharon Ziegler, Volume 4, pp. 85-86
“Reb Moshe includes toothpaste in the category of memareiach. Not so according to Rav Soloveitchik, who, based on a beraita in Shabbat 50a, which states that one may rub utensils to shine them on Shabbat, except for applying tartar to silver utensils, says that by the issur of memachek, it is prohibited only when the polishing will remove some layer of the substance. When brushing teeth, we are merely removing some particles or plaque from the tooth surface using a viscous substance that does not have enough density to hold together as a mass, and no shred of enamel is being removed from the teeth. Also, using toothpaste with a toothbrush is merely applying pressure to a substance and thereby flattening it, which is not memareiach.”
Thus, Rav Soloveitchik, zt”l, also held that one violates memareiach only when the newly smoothed layer remains on the surface. Since toothpaste is immediately washed away, leaving no residue on the surface of the teeth, there can be no issue of memareiach. When Rabbi Soloveitchik was told by a student that toothpaste advertisements claim that the toothpaste provides a
protective coating lasting for twenty four hours, he said that he was not convinced that there was any truth in the advertisements. Furthermore he pointed out that even if the advertisements were true, an invisible layer is not substantial enough to be recognized by the halachah (AhS YD 83:15 makes this last point).