high suction

I'm totally confused by what I witnessed last night when skimming one of my walls. I am laying onto old sand and cement render, I made sure to do two coats of PVA to reduce the suction as much as possible. When I started applying my first coat I suddenly had what looked like random patches of...

Okay so it's an absolute minefield out there at the minute with all the different products out there. I just want to know what is your preferred method when it comes to dealing with high suction backgrounds. Wickes bonding agent, PVA, SBR, Blue grit, gypPrime, bond-it. As I'm aware it does...