Why can you scrape paint off?

[Bahco]

My conservatory wall needed painting so I gave it a coat but was unhappy with the finish. I found that the old paint was able to be scraped off in chunks as if it was 2 mm of plaster.

It seemed that the previous coats had not stuck to the plaster (done about 2 yrs ago) . Is this because the wall had been PVAd and what should I do to rectify.
Thanks

 

[Bahco]

Here are some pics

View media item 5227 View media item 5228

 

[noseall]

it could be that there was a barrier like pva that caused the paint to lift.

..or, the paint may have been applied too thick and the suction in the plaster was too severe and it simply has not penetrated.

lacking a mist coat perhaps......?

 

[Bahco]

Hi Noseall

Think it was the PVA too. I am gonna sand down and mist coat it.