Preparing wood surfaces for painting can take, depending on the condition of the timber/woodwork, quite a lot of time and will require a lot of patience. The list below gives various tips on how to prepare wood surfaces dependent on their present condition.
New timber should have any knots sealed with knotting compound
to prevent the wood resin from staining the paintwork in the future. You can buy knotting compound (sometimes called knotting solution) from most good DIY stores for around £5 to £6. To apply, use an ordinary paint brush.
The timber should be lightly sanded down, cleaned off with a damp cloth and then a coat or two of wood primer applied prior to the top coat.
Previously painted timber/woodwork in good condition
If the timber/woodwork is in good condition it will probably only require cleaning with warm water and soap, or sugar soap if you prefer (sugar soap contains a degreasant for cleaning painted surfaces and is generally recommended by most decorators). It comes in a powder form that is mixed with water, or as ready to use wipes.
As with any cleaning product, I suggest wearing protective gloves when using it.
If the timber/woodwork has slight damage such as dints and holes in it, these can be filled using a good wood filler and sanded smooth.
Old or unsightly painted timber/woodwork
Safety tip :- Chemical paint stripper and heat guns should only be used in a well ventilated area due to the fumes that are given off, the area should be kept clear of children and pets. Paint scrapings should be disposed of as soon as is practical, gloves and goggles should be used at all times when using these methods to remove paint. If you suspect the paint you want to remove contains lead then I would try and obtain a DIY lead test kit, failing this I would remove the paint using a chemical paint stripper rather than using a heat gun.
If the timber/woodwork is covered with a zillion coats of old paint then the only solution may be to remove it completely using either a chemical paint stripper or a heat gun in order to obtain a good surface on which to apply new paint.
Chemical paint strippers are straight forward to use. Simply apply the paint stripper to the painted surface with an old paint brush, and after a short while the paint will start to bubble and peel. It can then be removed using a scraper, taking care not to damage the surface of the timber/woodwork.
For those awkward areas (spindles on stairs, mouldings etc), you can use wire wool and a wood chisel to scrape paint off. The timber/woodwork surfaces will need to be cleaned with warm water and soap, then left to dry completely before any primer and paint is applied.
Heat guns are relatively inexpensive tools to purchase and work by heating the paint surface up until it starts to bubble and then can be removed with a scraper, again taking care not to damage the timber/woodwork.
Be careful not to scorch the wood or set fire to anything combustible in the work area. Heat Guns usually come with clip on guards that protect walls and glass in windows and doors etc, from the heat.