However, there are situations where the previous coats of paint need to be stripped off:

• If the paint is damaged, peeling, pitted, badly chipped or crazed, repainting will not achieve a satisfactory or long-lasting finish.
• Where you want to go for a 'natural' wood grain finish.
• A heavy build-up of paint on the closing edges of doors and windows may be causing it to stick and another coat of paint will just make it worse.
• The more coats applied to carved architrave or other moldings can cause the original carving to become 'dulled' or disappear completely.

There are three basic ways of stripping paint from timber: by mechanical, heat and chemicals. Each has its own advantages and disadvantages - sometimes a mixture of methods may be appropriate.

WARNING: In older properties, the paint may contain lead . Added care must be taken if there is a chance that you are stripping lead based paint - avoid mechanical and heat methods of stripping (these could produce lead dust or fumes) and wear an appropriate breathing mask. Seek guidance to ensure the health of you and your family is not effected.

Whatever method you are going to use, it is always worth while trying it out on a 'hidden' area first to make sure it works.

Mechanical

Hand sanding

This method is only suitable for complete stripping where a very thin coat of paint has to be removed.

Start by using coarse sandpaper and as the grain begins to appear change to medium grade. On flat surfaces, wrap the sandpaper around a wood or cork block. Even with a thin coat of paint you will need plenty of effort.

To reduce the amount of dust produced, you can use wet and dry sandpaper with a wetting of water, but avoid this if you are intending to varnish the wood to show natural grain, the dust paste will be absorbed by the grain and its removal will require a lot more dry sanding. When wet sanding is undertaken, ensure the wood is allowed to fully dry out before the final dry sanding and painting.

Hand sanding is not really satisfactory for carved or shaped timber as the action of sanding will tend to flatten the detail no matter how careful the sanding is undertaken.

Mechanical sanding

There are various sanding attachments available for power tools.

Circular sanding discs mounted in an electric drill are not easy to work with and there is a high risk of scoring the wood. If the wood is damaged in this way, you will have to do a great deal of repair work before you can repaint it. Careful use of a disc to remove the initial layers of paint can make the job easier but don't expose the wood grain with a disc.

A Sanding Drum is a fast and effective method of stripping paint. The drum comprises a foam drum with an abrasive belt fixed to it. The belt should be used along the grain to avoid any scuff across the grain of the wood. The drum sander is quick and efficient, but has the drawback of producing large amounts of dust. Also, as it only move in a single, linear direction, there is a risk of score marks on the surface.

Sanding drums are available with various grades of abrasive (coarse to fine) - use the appropriate grade for the stage of sanding. Start with a coarse grade and change to a fine grade as the grain becomes exposed. Do not use one grade, the job will either take longer than necessary (if only a fine grade is used) or the finish will be disappointing (if only a course/medium grade is used).

As with hand sanding, the sanding drum is not really satisfactory for carved or shaped timber as it tends to flatten the detail.

The Orbital Sander power tool is 'more' gentle (and that also means slower) than the Sanding Drum, again change the grade of abrasive as you strip the paint, and again, it is only really suitable for flat surfaces. Keep the sandpaper free of dust and keep the sander moving, the small circular movements of the sandpaper can give a very good finish to a flat surface.

Also available is as a power drill accessory is the paint and varnish remover (a metal disc with perforations punched through - sort of like a cheese grater). This is suitable for removing the top layers of thick paint but must not be used once the wood grain is exposed as the grain will be damaged.

Always try to take work pieces out-of-doors when hand or power tool sanding, this will reduce the mess and the improved ventilation will keep dust problems to a minimum. Remember that any air borne dust will travel from room to room within a house, and, if working indoors, you will be unlikely to restrict dust to just one room.

For personal safety, always use a dust mask and eye protection - this applies to everyone in the immediate area, not just the person doing the sanding.

Using heat to strip paint

Today both blowtorches and heat guns are available to strip paint, the former using a naked flame to heat the paint while the latter uses just heated air. Whichever tool you use to heat the paint, you will need a number of scrapers to remove the paint, various types are available, straight edge, convex, concave, pointed to suit the timber profile being stripped.

With a blowtorch, the flame can be adjusted to give various sizes and temperature of flame, heat guns usually have one or two temperature settings, and the size/shape of the airflow can be adjusted normally by fitting different nozzles. Before beginning stripping paint with either tool, it is worthwhile getting the feel of the blowtorch/heat gun by practicing on a piece of scrap painted wood. Hold the tool at a constant distance, (generally about 150-200mm (6-8in) from the paintwork). Move the tool back and forth across a small area and the paint should start to wrinkle and then bubble, it is then ready to be scraped off. If the paint sticks to the adjacent paint rather than easily lifts off, play the flame/hot air over the area again and scrap again.

There is always a risk that the flame or hot air will scorch the wood by being concentrated on a small area for too long, keep the tool moving so that it is the paint which gets hot rather than the underlying timber. Be very careful when using heat to strip paint new glass (such as in a window frame), if too much heat is applied to the glass, it can easily crack.

Application of heat will never produce a stripped surface which can be painted straight away, you will always need to sand the exposed timber to get a satisfactory surface. Heat can cause the release of resins from the underlying timber and also for any filler in the timber to become detached.

Remember that the flame or hot air can cause damage around the area of application, keep away from plastics (such as guttering) and fabrics (such as carpets and curtains). Wear a pair of heavy-duty gloves and protective goggles.

You will always need to sand the stripped timber before painting, use coarse/medium/fine sandpaper as necessary.

Using chemicals

Most areas of UK have companies specializing in stripping complete items by dipping them in a tank of chemical solution, while obviously not practical for skirting boards or other timber fixed to the structure of a building, these facilities can be ideal for pieces of furniture or doors. However, be warned, sometimes the glue used in old furniture will not stand up to the process and you could end up with a 'self assembly' kit of pieces without instructions !!

Chemical stripper available for DIY use can be fairly expensive, so their use may need to be restricted to molding and shapes where their use is unsurpassed. Chemical strippers work best on thin coats of paint where one application can remove the paint back to bare wood. Thicker coatings may require two or three applications.

Two forms of chemical strippers are widely available, a gel type liquid which is applied by brush and a paste type which is laid over the paint to work its wonder. Whichever type is used, remember that it is a form of corrosive, so if you cannot take the piece of work out of doors, protect the surround decoration to avoid accidental splashes and drips.

Gel type is normally applied using a brush, usually requiring an initial thin coating to start the process, then followed by a generous application. After a minute or two, the paint surface will start to shrivel and it can then be removed using a paint scraper. As the stripped paint is rather sticky, wipe the scraper on old rags or newspapers to keep the scraper as clean as possible. Use the appropriate scrapper for the surface being stripped (i.e. straight, convex, concave or pointed).

Keep the scraper as upright as possible to avoid it digging into the wood and damaging the surface.

After the first application of stripper has been scrapped off, inspect the work piece, if large areas of paint are still in place, repeat the process with another application of chemical. Be aware that some types of paint may not be affected by the chemicals in a particular brand of stripper - in this case you may consider using an alternative brand or mechanically removing it.

The paste type of stripper is available 'ready for use' or as a powder requiring the addition of water. Whichever type is used, when the paste is ready for use, apply it to the painted surface using a putty knife, take special care to ensure that it is applied right into the bottom of any carving or other recesses. You cannot see the chemical reaction taking place, so you have to be patient and wait the specified time before pulling the paste away - try a small area to begin with just to check what has happened, the coats of paint should come away with the paste.

No matter how careful you are when using chemicals, there will always be small pieces of paint left on the surface, some may be so small that they are not obvious to the eye. As a final stage, apply a thin layer of stripper to the surface and carefully rub over using medium or fine wire wool. Only use small pieces of the wool at any one time and keep turning/replacing it so that a clean area is always presented to the surface.

A surface stripped with chemicals needs to be neutralized before applying fresh paint. So, using a constant supply of clean rags, thoroughly wipe down the surface with white spirit or the appropriate solvent recommended by the manufacturer of the chemicals. Where the manufacturer recommends water to neutralize the stripper, this often indicates a 'less harsh' stripper and the use of water can raise the grain of the wood requiring addition sanding before painting can be undertaken.

Whenever possible, always start and finish the stripping in the same day and don't let the surface dry out - the chemicals will penetrate the surface of the wood and if it dries into the grain, it will become harder to remove/neutralize.

The stripping chemicals may affect some fillers used in the underlying wood, so these may need to be replaced before painting can commence.

The chemicals and fumes may be harmful so always follow the manufacturers recommended safety precautions - normally these include only working in well ventilated areas, wearing gloves and goggles.

from:  http://www.diydata.com/techniques/strippingpaint/strippingpaint.htm