Traditional whitewash is a wood preservative -- a mixture of lime and salt that soaks into the wood pores and prevents rot and insect infestations. Whitewash also happens to look attractive in many decor schemes, so even though people rarely use traditional whitewashing anymore, they often create whitewashing effects with white paint. This type of whitewash doesn't have to penetrate, so you can use it to brighten up or -- if they are already too bright - tone down your painted walls.
Choose the Paint
Traditional whitewashing imparts a dull, washed-out patina to the wood surface, and if this is the effect you're after, you should use flat paint to make your whitewashing solution. Some decorators recommend using white primer, and there are advantages to this approach. Primer is formulated to bond strongly, it has a high solids content and when it dries it resembles real whitewash more closely than paint does. Its strong adhesion characteristics also mean you may be able to avoid covering it with a protective clear coat, thereby preserving its chalky appearance. Use high-solids shellac- or water-based wood primer -- not PVA drywall primer.
Mixing the Whitewash
Because you want to be able to wipe off most of the whitewash you apply, leaving only a thin, whitish overcoat, you need to thin the primer. One recommendation is to thin it with three parts water, but it's best to determine the optimum dilution by experimentation.
Open a can of primer, stir it well with a stir stick and pour some into a separate container.
Add three times the amount of water, or dilute the mixture according to your own specifications.
Test the whitewash on an inconspicuous part of the wall and adjust the dilution by adding more water or primer as needed.
Using the Whitewash
Create a whitewashed effect by spreading the primer onto a section of the wall with a roller and wiping it with a sponge or rag before it dries. If you're looking for a fairly heavy whitewashed layer, let the primer dry for 10 to 15 minutes before wiping it. Wipe sooner for a thinner layer. You can also adjust the transparency of the whitewash coat by adjusting the dilution of the primer.