Although I have read all the guides this step escaped me, I had understood that that type of cleaning included, for example, wrong alignment parts, I actually hadn't understood that you could exclude faces that you don't want to modify. I'll try to understand how it works, thanks. If I have problems with its functioning, I write here.
To selectively modify one face and leave the other unaltered during conversion, use face detection with unique identifiers and implement conditional processing.