re:shabbat soap

Question

re: using soap on shabbat being memachek. Rav Moshe Feinstein clearly held it was assur in his Teshuva (Orach Chaim I, Siman 113). It seems to me that this would be based on the Gemara Shabbat 75b, about smoothing skins (or the ground, according to Rashi) between the columns. Although it is true that no one uses soap in order to make it more chalak, it is impossible to use soap without this consequence, so it would be assur on that account. What do you think? --Aron