On Friday, 13 May 2016 20:58:55 UTC+1, wrote:
Could the vertical faces of the beam be plasterboarded and skimmed,
but the underside of the metal painted without being plastered?

Depends ... the beam might have to be encased to maintain structural performance in the event of a fire. If you need an unencased beam for aesthetic reasons you'd need to have specified that before specifying the beam.

Owain