Derek, inside the house, a keying agent is fine. That's how we plaster direct over tiles if we need to.
Exterior........ it's risky. The paint may well be stable. The trouble is, the bonding agent will ensure the render sticks to the paint - no problem.
Then .......... the paint starts to leave the masonry, and the whole lot is on the floor.
The simplest form of keying the surface is to just attack the paint with an angle grinder with a normal diamond disc.
Plenty of cuts with the grinder, and you at least have some mechanical key.
Then .......... PVA bonding agent, and off you go.