New Plaster
Thin the first coat of paint that you use down with upto 15% water or solvent, depending on the type of paint that you use.
Water stains
If you have a leak or a damp problem, there is no product that you can use to simply hide it. The only real solution is to solve the problem. If it is a leak, get a plumber in. If the damp is caused by humidity in the air and it is not a problem that can be solved by plumbing expertise then I suggest you use HG Moisture Absorber, which uses moisture absorbing granules to bring the humidity to an acceptable level. If you cannot solve the leak problem then a last resort would be to use G4 Damp Seal, which is a single pack urethane which will form an additional membrane on the surface to prevent the water from penetrating through.
Once the main problem has been solved, and the only remaining problem is a nasty stain left by it on the wall or ceiling, we can resolve this fairly easily. First, clean as much of the stain as you can, then apply two coats of Zinsser Cover Stain. The walls surface should then be ready to paint.
Fire Damage
Use Zinsser Cover Stain as above.
Flaking, Crumbly & Powdery Walls
Use Zinnser Peel Stop which glues down all lose surfaces.
Dark Coloured Walls
For this, you simply have to apply several coats of the finishing paint.