Hi, i will be having my wall skimmed before painting. I have stripped 90% of the wallpaper but there is an area above the window which seems to have some older layer of wallpaper which is 90% very hard set for 50 years paste with just a thin film like covering of old paper. Cant seem to get the fil off at all without digging at the underlying set paste which is invariably causing small damage to the plaster underneath. I have used a steamer and had it soaking wet but no joy.

Since i am having it skimmed could the plasterer go over all of this and cover it up or will he be expecting it all off and a smooth surface. Additionally, if it is being skimmed i dont need to sand it do i? Getting to the end of my tether with it

Pictures attached. Thanks in advance