Correct use of PVA when when patching render/skim over mesh.

[Draughtsman]

I would like to prevent edge contraction cracks when patching in over mesh (over cracks/helical) by using pva, left until tacky before applying render etc? Would this be before render and again before multi? What strength of pva? What ratio render mix? Do I need special sand (I just tend to use small bags of 'Builders sand' from TP).

Thank you.