If your going for a masonry approach Get a wire brush attachment to fit a 4"grinder, give surface a really good going over, mix SBR with cement to make a slurry. Paint over, as it is just drying (turning dark blue) give it a tight coat of s@c 4&1/2 sand 1 cement ,and set beads. Top with
5sand 1 cement 1 lime.
You'll have no suction as you've sealed the substrate so let scratch cure
If you don't want that membrane after the jobs done let me know my old dear needs some for her back garden also in regards to your post I know f**k all about rendering I'll just stick to plastering indoors once this chink flu has passed