What Goes On Before Paint

From DIYinfo.org

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Most interior and exterior surfaces require a preparatory coating before you can apply any paint. A new plaster wall, for example, will simply absorb paint if it has not been coated with primer.

If a painting surface is stable and sealed, the decorating will be successful. Be sure to choose a coating suitable for the surface.

Take special care with any pain­ted surface that will be exposed to the weather. The end grain of wood must be thoroughly soaked with primer to prevent the rain from penetrating.

Primer by itself is not particularly weather resistant, so do not leave a primed surface exposed to rain and wind for long. Cover it with an undercoat and a topcoat as soon as possible.

New windows, doors and other wooden fittings are sometimes supplied ready primed, Check for scratches and other damage and prime any areas that have become exposed. Give a second coat of primer to any areas that will become hidden by brickwork after the fitting is installed.

Choosing Primers and Other Sealers
Coating	Where To Use	Description	Advice
Knotting	Resinous areas, especially knots in wood	High in shellac. Prevents resinous areas in wood from discolouring paint.	When dry, coat knotting with primer.
Primer	New or exposed wood, plaster or metal.	Seals pores in absorbent surfaces. Forms a key to which other coats grip.	Buy primer for a specific surface or use all purpose primer.
Primer - Sealer	Stained walls and plaster; old bituminous coatings, areas treated with preservatives.	Contains fine scales of aluminium. Forms a barrier to seal the surface.	Apply a second coat if stain is still visible after the first.
Cold Galvanising Paint	Damaged galvanised metal, new iron and steel.	Zinc-rich coat which acts like a primer and prevents rust.	Stir well before use. Leave to dry thoroughly before applying paint.
Stabilising Solution	Mainly on external rendering which has become powdery.	Binds together surface to provide a firm support for paint.	Apply fungicide before stabilising solution on a mould affected surface.
Fungicide	Any affected exterior or interior surface.	Chemical used to kill mould, algae, lichen and moss.	When spores are dead, brush them away and apply another coat.