skimming walls...preparation?

[jungle_jim01]

With winter coming it's onto the inside jobs! I've scraped all the loose paint in the lounge and chipped off all loose plaster. (It's not lime, but for convenience sake have opted to just skim over the top with ordinary plaster). There's approximately 90% exposed plaster and 10% secure paint. I've read various recommendations about pva layer, but everyone seems to have a different ratio...also someone suggested using a scutch hammer to provide a key but i can't find much suggestion of this online. Any thoughts?
My biggest fear is skimming it and it all falling off a month down the line!

Thanks