Painting over old plaster after wallpaper removal.

[BEdwards]

A collegue of mine has removed the wallpaper from a room in his house with a view to painting it. Under the wallpaper is bare plaster. Is there any particular preparation that would be recommended prior to applying emulsion (Any particular primer/undercoat)? The plaster is probably original - 1930ish. As an aside, what is the actual difference between primer and undercoat? Is one oil based and the other water based? Do they both have the same use or are they intended for different applications?

Any help or advice would be grately received.
Many thanks,

Ben

 

[Zampa]

You have to be really careful here...the paste from the paper will have soaked into the surface of the plaster...painting over it will cause the paint to foam and bubble.

So make sure you scrub the walls thoroughly until every trace of paste has gone.

You odnt need a specific 'undercoat' just thin down the first coat of emulsion by about 20%

Use vinyl matt not silk, silk shows up every surface deviation...more so on old walls

Difference between primer and undercoat...primer is designed to penetrate the surface (in most cases) and lock on to it which when then allow following coats to be painted over it.

If you just use undercoat it will just sit on the surface and eventually flake off.

Undercoat is meant for blotting out the existing colur and keying on to the surface...its thicker and has rougher/courser pigment and different oils..the courser pigment 'etches' into the surface. and the oils are a type that bind to surfaces better than others.

They can also be water based primers and undercoats

Both do the same job