Is undercoat / sealer necessary for new plaster?

After having some plaster work done I thought there would nothing else to do but give the walls a few coats of white emulsion. However after the 3rd coat, I can still see a difference of colour from the old plaster to the new plaster.
I've still to do a couple of walls where there is old and new plaster on the same wall.
Is it worth putting undercoat on first? Would it make a big difference?
Also would it be worth while applying the undercoat to the walls I have previously painted that the new plaster can still be seen?
Steven.
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2 mist coats will suffice to seal new plaster when dry
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You should apply a "piss" coat first. So called because the paint has been watered down so much it is as thin as piss.
Basically, first coat, any cheap old emulsion mixed approx 50/50 with water.
I've had luck with some rooms where one piss coat and one proper coat was all that was needed, others needed a piss coat and two proper coats. (all these rooms done by the same plasterer).
One thing I've never got away with though is plaster coving. Always seems to need a gazillion coats regardless.
Hth Someone
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writes

Think I heard mention a while ago about sealing the plaster first with a 50 - 50 mix of PVA and white emulsion.
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diy-newby wrote:

There have indeed been mentions of using PVA (in various dilutions, etc.) on new plaster. Many of them also ask why the emulsion coats are peeling off...
As already said - piss-coat following by decent quality coat(s).
--
Rod

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Hi Steven
The others are all right. I think the problem lies elsewhere. Is the different texture between the old and the new plaster giving the appearance of a different shade? in which case you will need to go over it all with lining paper. Or maybe the existing paint on the existing plaster is leaching through the new and discolouring it, this would happen if it were yellowed for example with nicotene. Just some thoughts.
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Rednadnerb wrote:

Really it takes a LOT of paint, and heavily pigmented paint, to get the same color on new plaster as old.
One reason I use farrow an ball almost exclusively. Even THAT takes three coats tho.
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Thanks folks for the suggestions.
I think your suggestion above about the texture being different, which makes the new plaster appear a different colour is spot on. After 4 coats I think the room is now acceptable. Although I can still see the difference, any visitors I have asked have struggled to notice it ;o)
Cheers again
Steven.
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