Hi
if putting porcelain or marble tiles onto newly plastered walls do i use an acrylic primer?

First thing I would do is get the scales out.
Plaster is only specified to hold 20kg/sqm and most porcelain and stone exceeds this limit.

But to answer your question, use an acrylic primer diluted according to the instructions on the bottle, then use a flexible cement based adhesive.

Daz

As above, your tiles more than likely will exceed the weight for the plaster..

if it does exceed 20kg/sqm what can i do ?

What room are you intending to tile?
Bathroom, kitchen etc..

Your best bet would be to overboard with cement boards, glued with cement adhesive and also "plugged and screwed".

Daz

porcelain tiles on bathroom walls, these are very heavy.
and marble tiles in downstairs toilet.

Are these stud walls? If so, just rip out the plasterboards and replace with Hardie 500 (12mm thick) or Aquapanel. If not, then you will be best to fix Hardie 250 (6mm thick) on top as suggested above.

Daz

You could allways go for a lighter tile, ceramic perhaps..