New paint wont stick to previously painted plastered wall?

I bought a few small test paints from my local store, last night i tried to use them on my bedroom wall. At first it was going on well, untill i tried to touch up what i had done a few minutes previously. When the brush touched the wet paint it just removed it totally. It also looks quite scabby now its dried in.





I think the people who lived in the house before me used a washable paint (its very smooth to the touch), will i have to sand down the old paint or would an undercoat or primer do the job?





Thanks for your help.New paint wont stick to previously painted plastered wall?
If your paint won't stick it's likely due to using latex or acrylic base paint over oil base. The combination causes bonding issues, the latex won't stick or bond. To test the base use nail polish remover and rub on, if it comes off it's latex, if it stays on it's oil. If you find it to be oil, you can repaint with an oil base in a pearl or satin finish or paint in latex but you would have to use a good primer sealer as an undercoat. Do not paint with flat or eggshell sheens as these two are not washable nor durable. Sand off the scabby part and repaint following the steps above.New paint wont stick to previously painted plastered wall?
If it's in the kitchen, it could be that the walls are a bit greasy. Wash them with sugar soap and then use an undercoat. Did you use one of those paints that only needs one coat? They are notorious for going scabby as you described. Use ordinary paint from a reputable brand.
A primer should work.
you have to sand it down then mix plaster with white elastomeric paint and apply it to seal out any moisture from the wall, and serve as your basecoat as well. any waterbased acrylic paint will bind well with it....