How do I stop patches from showing through paint?

Question

I've painted a patched, bare plaster wall with a medium purple claypaint (Earthborn, "Jack Horner" color). The few places where I've patched the wall the paint is significantly darker, even after a mist coat, a full coat, and two further goes at spot painting those areas. It seems to be an absorption issue, as each time the patched areas took far, far longer to dry than the areas around them. How can I remedy this?

Below is a photo I took a bit after spot painting one of the patched areas with a roller. The new paint directly on the patched cracks is still quite wet, while the paint to each side is nearer to being dry.

Answer 1

I fixed this the hard way, by sanding off the impermeable filler. I was kind of a pain, but did get me the desired result:

For reference, the filler was Everbuild Deep Gap Filler. It's worked well for me elsewhere, but seems quite incompatible with clay paints, for what it's worth.

Answer 2

Use a dedicated primer. It might be best to do 2 coats around the patched areas. Another coat of the purple paint might do the trick on its own, but why risk it? A coat or two of primer will cover a multitude of cracks and patchwork. They even sell stuff now that is pink when wet and white when dry so you can tell where you've primed.