pva bonding

I will soon be skimming the ceiling of my en-suite and spotted a few comments about using SBR instead of PVA prior to the skim. Previously i've only ever used PVA, but then again i've never skimmed an area that is affected by humidity. I have both products in the garage and so is SBR the better...

Hi. I need to build up a level on an existing plastered and emulsioned wall. The white area in photo. Only about 3 or 4mm. Do I pva before bonding? Do I score the wall? Also do I need to pva where I’ve ‘dug out’ some cracks.