cross lining is the best way to go, if you try and paint old plaster and new palster on the same wall it will always show as a blemish/shade.
sand the walls, polybond, and cross line, coat one coat thinned out white emulsion, fill in joints in crosslining, touch up filler and two coats finish
it is not advisable to apply satin, or silk emulsion to plaster walls either matt is fine
but over crosslined walls u can coat with anything
u never need to strip either, as if in a few years u want to paper it, u dont strip it.