We've stripped a room intending to paint over the wallpaper.   After ringing a few shops to ask about
undercoats etc. we're thoroughly confused. Some say yes, some no - some say oil based ; others say water based.

Not all the wallpaper came off nicely and there was even other wallpaper under the wallpaper.  Some of that came off too.  It looks like someone was was going to strip that off but eventually gave up. I've sanded it all and filled any holes with Builders Fill.  It's all reasonably smooth to touch now.

In some places it's down to the gib and there's quite a bit of 'white stuff' (gib stop?) splashed around on edges and joins.

Can I just paint it now, or should a Primer, Sealer or Undercoat be used?  Which would be best to "smooth out' places where paper edges can be seen (even though reduced by sanding) ?

We're intending to use 3 coats and the paint is Guthre Bowron 'Wall & Ceiling Lo Sheen'.

Thanks for any advice.