Damp like marks coming through above skirting architrave and below coving

Painting on a wet or silicone based-flexible fillers cause that sometimes but the best way to hide it permanently it’s to repaint all affected areas with ( White or Grey Undercoat Paint oil based ) depending on the finish colour

If it's definitely not damp then zinsser b.i.n damp and stain blocker (shellac based) is the best on the market and will definitely work. Can be painted over in 20 mins. The only way water marks will reappear is if you still have a leak somewhere or water is getting in from outside.

The staining you are describing sound to me like historic water stains. Assuming that the source of the staining has been remedied then you need to obliterate the stains before finishing. I am assuming you are trying to emulsion the area? no amount of even the highest quality emulsion will obliterate a historic water stain. you need to seal the stain first. Undercoat (oil based) is NOT the way the to go as suggested above. a proprietory SHELLAC based treatment (also mentioned above) is the correct way to deal with this. ZINSSER B.I.N stainblocker is your best bet. around £25 for a 1 litre tin. Apply in thin coats allowing 15 -20 mins to dry before reapplying until the stain/s stop bleeding through then refinish in the desired emulsion / colour of choice.

If you have blocked up the chimney, have you capped the chimney pot? If not, you need to cap and vent it and install vents in the chimney breast. Otherwise the old soot inside will get damp via condensation and further staining will seep through.

Hi it may not be damp as such but there is water ingress from somewhere, its best to investigate this further before masking the issue, also the fact you have took a chimney away has reduced the ventilation in the room aswell which can lead to condensation problems if the air flow in the room isn't adequate, the recent cold snap could be the issue also, as if there's water sitting in the wall when it's cold it freezes an can break the masonry internally then flow more freely when it thaws again, check the outside of the house aswell for any telltale signs externally where the wet patches are inside , i.e broken gutters, cracks, moss growing on wall, pipework freezing and breaking joints could also be the issue, its best to identify what it is before covering potentially a small problem only to find out a year down the road its become a major issue, thanks.

This needs multiple coats of zinsser BIN or 123. Once the mark has disappeared it will need the matching emulsion colour on top.

Is it near a window if so are windows sealed,bedded etc water could be running down cavity.which will cause damp. Do you have vents in windows or walls sounds like ventilation problem