We have stripped the wallpaper off the walls in our house - these are
what we call gibralter board, what the Americans call drywall or
sheetrock, I don't know what it's called in the UK. Now we will hire
someone to skim them with plaster (to be finally painted). The spatula
used in removing the paper has left some gouges, with occasional little
flaps of drywall paper sticking up.

One of the tradesmen who has given us a quote says that the walls must
be sanded first to smooth off the protrusions, then painted with an
oil-based sealer before the skimming. Another says sanding is not
necessary, he proposes to paint with sealer then start applying the
plaster. In both cases the plaster will be applied in two or three layers.

We are concerned about this inconsistency. Is it necessary to sand
first? I would have thought the little bits of paper sticking out would
either require extra thickness of plaster to cover them, or would tend
to come through to the surface and maybe create irregularities, perhaps
responding differently to paint. How thick would the total plaster
layer be normally?

Thanks in advance for advice.