Masonry paint epic fail

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Hi
I’d really appreciate any advice from the experts on this one. I’ve self built a 2 storey extension and the front elevation is rendered and painted with sandtex smooth white masonry paint.
When first painted, 3 weeks after rendering in November, it was fine. Recently I found a single patch where the paint bubbled and came off.
I thought not great but left it alone to plan to paint over it when the weather improves.
Today (after cold snowy spell) the weathers warmed up a lot, but I’ve noticed large areas mainly at the lower half of the wall where the paints come away and bubbled.
I’m absolutely gutted as I built the entire thing myself and the painted render was supposed to be a wow factor, and is now coming off and will look terrible.
I’d really appreciate any advice on where I went wrong, I suspect I didn’t give the render (2 coat sand cement) enough time to dry before painting, and also how best to fix this , scrape the bubbles patches off, let it dry in warmer weather and repaint? Or any other product I could use.
I didn’t stabilise the new render, just brushed it down and painted 3 coats over a weekend.
Thanks for reading, all thoughts appreciated, pics attached
 

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Three weeks is not long enough to dry out especially in winter time. A few things, did you mistcoat it?, this is essential on new render. Did you put waterproofing in the second render?, you should only waterproof the scratch coat. I would leave it until the weather warms/dries up a bit before doing anything else, then remove any loose and flaking paint, mistcoat the bare patches with slightly thinned (10% water) masonry paint, and apply two full coats allowing enough time to fully dry between coats.
 
Cheers for the reply
I didn’t mist coat, I am sure the sandtex instructions didn’t say to either
But I’m sure you’re right that it’s a good idea to do it, I’m just a bit miffed that I followed the manufacturer instructions (I think) but got it wrong
Yep only the scratch had waterproofer in
Thanks again
 

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