Hello. We recently bought a 1960s house and started redecorating room #1. We've ended up with what I think are referred to as spider cracks, in multiple spots. I'm pretty sure it's not the paint that's the issue (see below). We're hoping someone help identify a remedy. Here's the info:
* The plaster wall had been patched in multiple spots by the previous owner(s).
* The cracks in the paint appear in multiple spots, but not everywhere (there are large areas of wall completely unaffected)
* I'm pretty sure the cracking is only happening over patches that were plastered by the previous owner, for various reasons: to the naked eye it definitely looks that way, we also tried matching up some pre-painting photos to the current problem areas and it looks to be a good match. I can also see that the same cracking has happened in other rooms in the house where paint has been applied directly on to the plaster walls.
* I re-plastered additional bits of the wall in room #1 myself, and the cracking doesn't appear to be happening over those patches.

These are thick walls, not drywall. The walls in this particular room were papered when we arrived, but underneath they were bare - no paint, just bits that had been plastered.

We stripped off the wallpaper, washed and scraped the walls, then sanded them with an electric sander to smooth them out and try to ensure we got as much of the wallpaper paste off as possible. I guess that would be enough to rule out paste residue as being the issue?

We applied 2 coats of primer/undercoat (http://bit.ly/11F60Db) and 1 coat of water based emulsion (http://bit.ly/1HOBgR6) but halted the work when we noticed the cracks.

Here is a picture of the cracking - although it looks to be affecting the most of the wall that's just the cropping of the image there are much larger areas unaffected http://bit.ly/1HOCmwj