Dark lines appearing on painted outer wall of extension

Hi there, It sound like you have a problem with an ingress of damp, or have in the past. Just decorated to cover up and these dark patches wont resolve this issue as the damp issue behind or within the wall and ceiling would need attention to stop continuing problems/damage. After this has been done and depending on the extent of the damaged, and all areas effected have been correctly dried out. I would recommend applying a couple of layers of stain block. Leaving each coat to dry properly before applying at least two coats of your final finish (i.e emulsion). In doing this you have firstly resolved the damp issue which will eliminate any further problems in the future, or stop damaged areas increasing in size. The stain block will then create like a barrier to hold back any further color disfigurements once the final finishes are applied. Hope this helps Kind Regards Aaron - AW Decorating

these dark lines of yours are where the doorway has been blocked up where the walls join correct .if so they are airline cracks which always show up like thin pencil marks .its where the new brick is joining the old.the only thing you can do with that is fill it with a none shrinkable filler .