Which undercoat/primer?

[tntcoder]

Hi,

Having removed wallpaper and sanded down everything, i'm left with a wall that is half an old plaster skim and half old masonry paint.

What's a sensible base coat / primer to put on this? I assume I need something for the old skim plaster?

I was considering Polycell Basecoat?

Thanks!

 

[mattyg]

B&Q cheap white emulsion. The stuff that seems to come already watered down.