Can I use pva sealer on a wall which is already painted?

Yes you can. All the PVA will is bond to the paint you already have on the walls and will help the new coat of paint bond as well, Just make sure any flaking paint is taken off if any. You may want to water the PVA down with water.

Anyone using pva with paint or paint on top of pva is not professional in any way. I know this question is a few years old but there's products that are made for sealing paint. There's nothing wrong with sanding any bad areas and emulsiong as normal unless you have damp issues. Decorators answer decorating questions and builders and DIYERS join DIY on a budget group on facebook

Sorry using PVA over a painted substrate is a not advisable at all. Unless the surface is chalky this will help bind the surface. If the painted surface of the wall is flaking, simply scrape the flaking paint from the wall. If the plaster is showing after scraping flaking paint, simply coat with a 1 part PVA and 5 part water mix to bare plaster then face fill with quality filler (toupret interior). The problem you will find painting over a PVA sealed painted surface is slipping of the paint or a crocodile effect. If the wall is an exterior wall use a good quality penetrating sealer followed by two coats of masonary paint.

Its not a good idea-rub it down well if its silk. fill the wall if needed. Its a no no unless you are plastering it

A wall that has been previously painted will not need pva.fill any holes and rub down wall before painting. I would slightly water down first coat to help adhere to wall and help with a smooth flow off your brush and roller better smooth finish.. then let it dry over night if possible room temperature not blazing hot with heating on..then apply a another need coat of paint for that strong colour effect

Sand down and use a product called Zinsser gardz if you want to seal the wall. Pva is no good