Best primer/undercoat for hiding imperfections

[graememac]

I'm preparing some plasterboard walls for painting. I have tried sanding the walls down but there are still patches where paint has come off and are uneven. Is there a good undercoat to use that will cover these imperfections?

 

[StudWall]

I got some filler paint from toolstation, was about £25 for 5l

it did cover up the minor imperfections (very minor) like pinholes in my ceiling but anything bigger than a mm it didnt help