But thou, when thou fastest, anoint thine head, and wash thy face;
Note 35 at Mt 6:17: It was a Jewish custom to anoint oneself with olive oil (De 28:40, Ru 3:3, 2Sa 12:20, and 2Ch 28:15), and in Jesus' day, it was apparently customary to anoint the heads of your guests also (Lu 7:46). Sometimes ointment or perfumes were used in this personal anointing (Am 6:6, Lu 7:38, and Joh 12:3). A lack of having one's head anointed was associated with mourning or sorrow (2Sa 14:2 and Da 10:3). Therefore, in this verse, Jesus was saying that there should be no external sign of fasting or sorrow, but it should be done secretly to the Lord to receive a full reward.
Paul stated in 1Co 7:5 that abstinence from the physical relationship in marriage for the purpose of fasting should not be done without the consent of your mate. Therefore, it can be concluded that a fast does not always have to be totally secretive to be productive. Rather, Jesus was once again dealing with the motives behind the actions. The fast must be directed toward God and not man. Compare with the motives spoken of in note 25 at Mt 6:1 and note 26 at Mt 6:3.