It is unlawful for a woman to uncover her face and hands in the presence of strange men whether she was in Salah (Prayer) or Ihram (ritual state for Hajj and `Umrah) or in other usual situations. This is according to what is narrated by
`Aishah (may Allah be pleased with her):
Riders would pass us when we accompanied the Messenger of Allah (peace be upon him) while we were in the state of Ihram. When they came by us, one of us would let down her outer garment from her head over her face, and when they had passed on, we would uncover our faces.
Abu Dawud, and
If this is what should be done in the case of Ihram when women are supposed to uncover their faces, it will be more urgent in other cases because of the generality of the Saying of Allah (Glorified and Exalted be He):
And when you ask (his wives) for anything you want, ask them from behind a screen: that is purer for your hearts and for their hearts.
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.