Ervin Pergjergji
Hi there, Not necessary to skim it or plastering unless the current plaster is damaged. Try Zinsser B.I.N Primer / Sealer Stain Killer, that is the best to cover any oil stains or marks on the ceiling/walls, It will dry in 15min and repaint over with bathroom paint after 30min, after you do this you shouldn't have any problem in the future, Hope this helps.
