Lining Paper

[Old DIYer]

Hi, This is my first post here so please excuse my mistakes.

I am redecorating my kitchen, have stripped off old paper, and am using lining paper prior to emulsioning the walls.
My kitchen has been extended, the old back door way leads through to the extension. No door fitted, the extension is new plaster and will not need lining paper.
How can I finish the edge of the lining paper in the old door way.
Do I just sand down the lining paper edge at the door way corner?

Do I need to thin my first coat of emulsion?
Hope you can understand my first question.
Thanks

 

[footprints]

Might sound silly but paper the lot, the paper will have a different texture when painted to the plaster.
Must say I use Dulux trade two coats and never worry about a mist coat on paper. If you paint the plaster yes a mist coat first.