Any decoration job, either DIY or professionally done is always only as good as the preparation and thought that has gone into it. If a surface is to be varnished for example, sanding the timber across the grain prior to varnishing will leave scratches which will show through the varnish and ruin your work. There is grease in the air in every home, from cooking and general cleaning. This grease settles on all surfaces and must be removed. Paint, if it to stay where its put, must have a stable base. We get so many questions as to why a surface is peeling or has gone blotchy etc and the answer is, 99% of the time, because the surface has not been prepared properly. Other paint defects can be seen in the decorating section of our tips and tricks pages.
New or bare timber will need to be primed after sanding if you are to paint it. Sand paper is available with different degrees of coarseness of the surface. These are called grades. The grades are measured in numbers and the lower the number the coarser the grit. New timber should be sanded with medium to fine paper of about 120 grade. After sanding there may be little indents which need to be filled. Nail heads etc can spoil your paint finish. A very popular myth amongst DIY enthusiasts is that paint can fill holes. This is not true! All purpose filler will usually do the trick, for varnished surface a wood filler should be used which matches the colour of the wood as closely as possible. For large areas an electrical sander may be ideal. Using rolls of abrasive paper makes it easy to cut to length to fit in most power sanders.
Where the woodwork meets the walls there is often a gap. This gap can be filled very easily using flexible filler or decorators caulk in a special application tool called a sealant gun. The sealed end of the tube is cut off, a nozzle is screwed on. This nozzle has a hole in the end which is, more often than not, not large enough. It can be cut back further to allow a larger bead of filler to be released from the tube. Cutting this nozzle at an angle makes application much easier. The tube is then inserted into the gun and the trigger squeezed to apply the filler. Once the gap is filled a wet finger can be run down the joint to "tidy" it up. Surplus filler can be removed easily with a damp rag. As with anything else in the building industry, practice makes perfect, so don't rush to your walls straight away, sacrifice a couple of pounds on an extra tube to practice with on some old offcuts. This little bit of preparation will save you hours of frustration later.
Primer is a thin paint which soaks into the grain slightly and takes some of the porosity out of the new, or bare timber. If an undercoat or top coat of paint is applied to bare wood, the moisture is "sucked" out of it too quickly and the paint itself does not get a chance to soak into the timber. The paint therefore dries on the surface and has no "roots" so to speak. It will quickly peel. The primer itself can only soak in properly if the surface is free from grease and the grain is a little open to receive it. This can be achieved by (if necessary) washing down with a solution of sugar soap which will degrease the surface and then sanding with a medium grit sand paper. The best sandpapers are aluminium oxide or silicone carbide papers which are a little more expensive but stay workable longer. It is always best to wrap the paper around a sanding block to keep a flat surface with which to sand. If the timber has different contours, such as chamfered architrave or skirting, wrapping the paper round a firm sponge will allow the paper to get into and onto the contours. Remember to protect carpets etc with dust sheets. If there are any knots in the timber these should be treated with knotting solution which will stop the sap leaking out of the timber and spoiling your paintwork.
For previously painted surfaces there is no need to remove all the paint if its is not flaking and is in a stable condition. Clean thoroughly with sugar soap, sand down with a fine paper to give a key for the new paint, and paint. An undercoat is always a good idea on previously painted surfaces as it will give you a much better finish and stops the paint "dragging" on the surface. Wear a mask for sanding existing paintwork as older paints contained lead and this can be dangerous. For the safest way, use wet and dry paper and keep it wet. If the existing paint is not in good condition, it may be as well to remove it. This is usually easiest done using a paint stripper together with a shaped shave hook or putty knife to get into corners and mouldings etc. Follow the instructions on the container and wear rubber gloves.
DIY Doctor, in the interests of safety does not recommend sanding down or burning off paint from internal or external surfaces of a house unless you can be sure the paint does not contain lead. If this is the case, please see our project on paint stripping where you will get full instructions on how to strip. The product on this page is the best product DIY Doctor has found in over 30 years of decorating, for stripping paint. It is safe and it is clean. If this project looks like product placement it's because it is ! This is the only way we strip paint and we know what we are doing.
If only small areas of the paintwork need to be removed, sand them down (providing you can be sure there is no lead in the paint) as smoothly as possible creating a very gentle slope between the existing paint and the wood underneath. Then spot prime the areas of new wood with primer and carry on painting with undercoat and gloss etc.