Prepare the surface

Carefully examine the surface you're about to paint. Although it's hard to notice on a day to day basis, interior surfaces accumulate dirt, fingerprints, and a variety of stains and contamination that dull and diminish your home's appearance. A coat of paint won't correct or cure these surface flaws, so you have to address them before you paint or they'll undermine the quality, look and longevity of your paint job.

Proper surface preparation often takes much longer than the actual application of paint. Be sure to allow for this important step in your budget and schedule.

          Remove fungus      Fungus is a spotty black, grey and brown mould which is sometimes stubborn to eliminate, but if it is not removed before priming, painting or staining, it can grow through the new coating and ruin your paint or staining job. Fungus thrives in warm, moist, low light environments; utility rooms and bathrooms should be carefully inspected. If it's present, treat the fungal contamination with a 3:1 mixture of water and household bleach or specialised fungicidal washes. Sponge the bleach or fungicidal wash on the fungus allowing a half metre margin around the affected area just to be safe. Allow it to remain on the surface for at least 20 minutes, and reapply as it dries. Rinse the area thoroughly with water and then wash it with a mild detergent and rinse again. Always wear goggles, rubber gloves and old clothes when working with bleach.
Never mix bleach with ammonia products; the combination can release poisonous vapours

          Clean the area      Before priming you should remove any dirt, chalk or treated fungus or algae. Scrub the surface area with detergent, water and a stiff bristle brush. Rinse the surface thoroughly, and allow it to dry completely before priming. Another effective surface cleaning method is power washing with water. Never use harsh cleansers since they can infiltrate the substrate and inhibit paint performance. If using bleach to remove the fungus or algae, be sure to wash the area thoroughly and allow to dry before applying paint. Hold the power spray nozzle 150mm to 200mm from the surface to prevent damage, and at a horizontal or downward angle only. Be careful around doors and windows since the spray could have enough force to shatter glass and damage seals. Follow all manufacturer's instructions. Wear goggles and water repellent clothing and footwear.

          Prepare glossy surfaces     New coatings do not adhere well to high-gloss surfaces, so it is recommended that all glossy areas be dulled for the best primer or paint adhesion. Use a fine grade sand paper. After sanding, remove any dust with a damp cloth.
          Chemical deglossers are also available, but require care and caution. Leaving the product on the surface too long can soften and wrinkle the old coating. Make sure there is ample ventilation, open windows and use fans to keep the fumes from building up in the room. Follow all manufacturer's instructions and treat these products as strong solvents, exercising safety precautions with protective goggles, gloves, respirator, and clothing.

          Prepare other unpainted surfaces    

New Plasterboard
Before painting materials like new plasterboard be sure all joints and patches are sanded smooth, and dusted with a cloth. Next apply a wallboard-specific sealer. Pay attention to the manufacturer's instructions and spread rates. As an alternative, a general purpose stain-blocking interior sealer can be used. If staining occurs with a wallboard sealer, apply a stain-blocking sealer.   

New Plaster
Allow plaster to dry thoroughly. With a simple patch job, this will take one to two days, for larger areas like walls and ceilings it can require a week or more depending on the weather and interior humidity conditions. Thick plaster applied in extremely humid conditions can take a month to dry adequately. Once dry, all rough areas must be sanded smooth with medium grade paper followed by a second treatment with fine grade paper. Clean surface of all dust with a dry cloth and then apply a stain-blocking interior water-based sealer recommended for plaster surfaces. Be sure to wear a mask and protective eyewear when sanding.                                                                               

Tile
Treat any suspected fungal contamination and inspect grout joints and repair them as necessary. Next wash the entire surface with a detergent and an abrasive non-metallic pad. Rinse and dry thoroughly. As an option to guarantee maximum adhesion, sand the tile surface with a fine aluminium oxide sand paper. Clean area off with a damp cloth. Use a high-adhesion interior water-based bonding primer. This type of primer requires the use of a respirator and ample ventilation. Allow it to dry thoroughly before applying a finish coat.          

MDF
Medium Density Fibreboard is a manufactured wood made from sheets of compressed fibres. It is highly versatile and can be used for structural purposes like eg. cupboards. This surface should be treated like any hardboard surface when decorating. MDF is capable of taking most paints.

              Prepare previously painted surfaces          

Inspect the surface for cracking, peeling, flaking, loose paint. Depending on the severity of the damage you may choose to employ one or more of the methods described here to prepare the surface for a new coating.

Scraping
This is a traditional effective approach for most flat surfaces from plaster and wallboard to wood and ferrous metal. Use shaped scrapers like triangle or oval shapes to get into corners and on rounded profiles. Take care not to gouge, score or otherwise harm the surface. Feather sand all rough edges with a medium, then fine grade sand paper.

Wire Brushing
Use a stiff wire brush, being careful not to damage the substrate. Once all loose paint has been removed sand the surface with a medium then fine grade sand paper.

Sanding
If the surface is just flaking slightly you can simply sand the area with progressively finer grade sand papers starting with coarse, then medium, then fine.

Chemical removers
Choose a paint remover that is recommended for the coating and substrate you're preparing. Apply a heavy coat of the remover with an old or a low-end natural bristle paint brush since the remover will likely ruin the brush.

Give the product plenty of time to work as recommended by the manufacturer, usually 15 to 20 minutes or longer depending on the thickness of the old paint. Carefully remove the softened paint using a putty knife or wooden blade and scrape the material into doubled paper bags inside a cardboard box.

Reapply more stripper if needed and then clean the surface with wadded up paper towelling, newspaper or a commercial pad designed for this purpose. Before using these products, clear the area of children and pets; cover floors and steps and remove plants, rugs and furniture.

Dispose of all waste after the job carefully following manufacturer instructions. Wear goggles or face mask for eye protection; use chemical resistant gloves, long sleeve shirt and trousers to protect the skin; and protect breathing with a respirator designed for use with chemical solvents.

There are low odour alternatives to the strong solvent removers, but they may take several hours to accomplish the same job. Once the remover has done it's work, sand the surface thoroughly with fine sand paper and dust before applying primer, stain or varnish.

          Prepare stained and discoloured surfaces    

Water stains
The first step to covering water stains is to make sure that their source has been eliminated. Once you are assured that no more damage will occur, begin the preparation process. Clean as much of the stain as possible from the surface and rinse. Allow the surface to dry and apply a stain-blocking solvent-based or water-based primer. If there is still evidence of stain-through, apply a second coat. Solvent-based primers or sealers are the most effective in stopping stubborn stain-through problems, but require plenty of ventilation and an appropriate respirator. Once the primer or sealer has thoroughly dried, apply the finish coat.

Tobacco, fire damage, soot, grease
Clean the stained area as much as possible with a detergent and rinse thoroughly. Allow the surface to dry and apply a stain-blocking solvent-based or water-based primer or sealer. If there is still evidence of stain-through, apply a second coat. Solvent-based primers or sealers are the most effective in stopping stubborn stain-through problems, but require plenty of ventilation and an appropriate respirator. Once the primer has thoroughly dried, apply the finish coat. In the case of fire damage, solvent-based primers or sealers provide the best combination of stain hiding and minimising odour.

           Prepare wallpapered surfaces    

It is far better to remove wallpaper and residual adhesives before painting. If not you risk a variety of problems from lifting and curling to staining and uneven textures. It is sometimes however necessary to paint over it. Here is how to handle either situation. Wallpaper can be removed by peeling, soaking and scraping, or steaming and scraping. Scrub off any glue and rinse thoroughly. Any gouges or scratches can be patched with plaster or filler, then sanded and dusted with a damp cloth. Interior solvent-based or water-based stain-blocking sealer should be applied prior to painting.

When painting over wallpaper begin by checking for loose areas of paper and poorly adhering seams. Re-glue these problem areas and then try a paint test in an inconspicuous area of the room to make sure it will look acceptable, making sure there is a seam included in the test area. Apply a solvent-based stain blocking sealer using ample ventilation and a respirator. Allow it to dry overnight and inspect it to ensure there is no discoloration and apply a second coat if necessary. Apply a finish coat to the test area and allow it to dry. Matt finish paints will minimise the texture of the underlying paper. Consider applying a second coat and then evaluate the test area to see if you will proceed with the entire room.

          Prepare bare wood     Inspect wood for fungus and if present treat as recommended in the "Remove Fungus" section. If the wood surface is rough it should be sanded smooth with a medium grade sand paper. Be sure to sand with the grain, never against it, diagonally or across the grain. Wipe away any dust with a cloth. To achieve the smoothest appearance, apply a coat of water to the wood with a wet but not soaking cloth. Allow it to dry 30 minutes to raise the grain of the wood. Sand with fine paper to remove the raised grain. This will avoid grain raising when applying primer, paint or other coatings. Use a stain-blocking primer applied in a heavy coat and allow it to dry overnight. A second coat can be applied if staining is evident. Solvent-based stain-blocking primers are most effective over high-staining woods such as mahogany and redwood. Light sanding may be required before a finish coat is applied. Follow manufacturer's recommendations for best results. When using solvent-based primers, proper ventilation is required along with use of a respirator.

           Dealing with the dangers of asbestos    

Before 1983 most fibrous cement sheeting and some textured coatings contained asbestos. Asbestos fibres present a serious health hazard if they are disturbed and inhaled during cleaning, Therefore cleaning or preparation for painting should be done WITHOUT sanding, or scraping so as not tocreate a dust. If the surface is sound, simply wash it clean before painting. If the fibrous cement sheet cannot be prepared or painted without disturbing the surface, before tackling the job contact the Environment and Health Department of your Local Authority, who will advise on the most appropriate way to proceed.

             Dealing with old, lead-based paint        

The paint on old houses or buildings may contain lead pigments. White lead was used in house paints until the mid-1950s, and red lead was commonly used in primers until the 1980s. Lead chromate pigments were used in some red, yellow and orange colours, and calcium plumbate pigment was used on galvanised iron and steel structures up to the 1980s.
Though not so common, lead test kits can be purchased from diy and specialist paint stores.
If possible, the old lead-containing paint should be left undisturbed and simply cleaned and repainted.

However, if you are removing lead paint inside:

resource: www..paintquality.co.uk