Flaking or peeling paint should be scraped, filled with a proprietary filler, and then spot-primed when dry.
Alternatively, the paint can be removed using liquid stripper (solvent or caustic) or heat (blow lamp or heat gun).
Cracks should be raked-out with a shave hook or similar tool to widen the crack and then filled with a proprietary filler.
This allows the filler to "key in" to the surface of the crack, which, in turn, prevents the filler from falling out.
Nail holes, dents etc. should be filled with a proprietary filler, or "stopping", allowed to dry, and then abraded.
The surface can then be spot-primed to kill the suction of the bare filler before painting.
Scratches and other minor defects, can be filled with fine-surface filler, allowed to dry, and then spot-primed.
Mildew should be removed using a proprietary fungicidal solution or thick, neat bleach, applied with a synthetic brush.
This should then be rinsed off, several times, with plenty of clean water & allowed to dry thoroughly before painting.
Efflorescence can be sealed with a coat of alkali-resisting primer or oil-based undercoat so that it does not reoccur.
Water-stains (ironmould) can also be sealed with a coat of alkali-resisting primer or oil-based undercoat.
Bitumus surfaces: cooking or nicotine stains can also be sealed with alkali-resisting primer or oil-based undercoat.
Corrosion or rust should be removed using a wire brush or mechanical tools (electric sander etc.).
Refer to the bare metal section for further details including the use of metal primers.
Grease should be removed using turpentine or white spirit applied by brush or cloth.
If this treatment does not cure the problem, wipe the surface with "fullers earth" powder on a damp cloth before painting.
Dirt should be removed using sugar-soap, following the manufacturers instructions.
A mild detergent can be used as an alternative to sugar-soap if necessary (rinse thoroughly).
*** IMPORTANT ***
Wood and metal surfaces, especially pre-1960, may have been decorated with paints containing lead pigments
The preparation of such surfaces can be hazardous