• Crumbling plaster

This needs to be cleaned with a scrubbing brush and water to remove all the crumbling and loose plaster. Repair the damage with an appropriate filler. Apply a primer for the job and let it dry thoroughly before painting or decorating

• Graffiti, pen or felt tip

There are primers available to cover these marks, allow to dry thoroughly before adding a second coat before painting 

• Grease, mildew & mould

The surface should be cleaned with a household cleaner first. Then clean the area with sugarsoap.The area then needs to be painted with a primer designed to cover these type of patches

• Old oil based paint                                                  

Some old paints used a  special pigment that tends to bleed through latex based paints. again buy a primer specifically designed to cover old paint and follow the instructions before painting or decorating over