Seal it with PVA and water etc.. stops the grout pulling any crap off the wall and going black and also the adhesive drying and maybe tiles falling off!!
Opinions on the use of PVA are mixed. You should really contact the tile adhesive manufacturer to see what they recommend - they might have their own primer.
You should be able to tile straight onto the skim. However a suitable primer will give you an extended working time as it stops the skim sucking the water out of the adhesive.
Check what your adhesive maker suggests, not always PVA.
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