Antonyms for the phrase "slapping on" would include "gently applying," "carefully spreading," "meticulously placing," or "thoroughly coating." Rather than hurriedly or forcefully putting something on, these antonyms suggest a slow, careful, and intentional application. By using these antonyms, one could convey a sense of precision, attention to detail, or even gentleness. Additionally, they could be used to suggest different moods or contexts. For example, while "slapping on" might suggest a rushed morning routine, "gently applying" might suggest a relaxing evening skincare ritual. Overall, these antonyms can help to add nuance and specificity to one's language.