Roofing Joist

[xmtr34]

Hi all. I'm new to all this forum stuff so sorry if I'm making no sense.
I am extending a loft space into an existing bedroom space for my son and on one side I'm leaving the roofing Joist exposed and the other side I'm covering it up, making the space a sort of a old / new type area. I have rubbed down the joist, leaving a bit of patina in the wood, and am now wanting to seal it, leaving the look without staining dark.
What would be the best oil, wax, varnish etc, to use for this.
Pictures attached of before and after.

Many thanks
Colin.

 

[piglet11]

Not technically a joist (a joist is a timber member that support the floorboards), but a purlin.
This is rough timber, not meant to be decorated.
For safety, I would take off the sharp edges of the timbers - a sharp block plane works well - and for sanding, use a coarse grit like 60 grit (yellow paper usually) designed for powered sanders.
If you don't mind some exercise, just make up a hand block with the paper. If not use a belt or orbital sander.
If you can get a reasonably smooth finish, I swear by Osmo Polyoil (Polyx).
As an aside, I'm presuming you are going to plasterboard the rafters.
I would take the opportunity, if not too late, to insulate between the rafters with something like 40-mm Celotex.
Makes a huge difference to temperature control and cuts out drafts.
Keep a 50-mm gap between roof felt and Celotex to maintain ventilation - it's then called a warm roof.