Any owner of an old house will tell you that it can take weeks to find the right doorknob or mantel-piece but only a few minutes or hours to install it.

In fact, the Preservation Workbook would like to propose the first law of old house care: The time devoted to getting ready to do a job will and should always be longer than the time needed to do it.

Painting the outside of a wood house is a good illustration. (What follows is for wood houses only. Unpainted brick houses should stay unpainted.)

When you properly apply paint to the wood, you are giving it the best protection possible against rot. What takes time is preparing the surface for the painting and eliminating any sources of moisture. five or 10 years. The job includes scarping peeling paint, priming the surface and repainting.

If the paint looks faded or dirty in less time than that, your best cure may be a gentle washing with soap and a scrub brush followed by a thorough rinsing of the surface with a garden hose. Many household mainteneance and cleaning companies will do the job for you.

Too-frequent painting can actually be harmful to an old house. If the paint film becomes too thick, it becomes a moisture trap.

An example: Several years ago, the owners of the Hammond Harwood House in Annapolis discovered the danger of too many layers of paint applied to hand-carved 18th century moldings on the doors. Over two centuries, successive layers of paint had hidden the moisture damage from leaks in the watershedding system of the house. In places the only thing holding the molding together was the paint. The wood was completely rotten.

A local craftsman, John Lee, had to remove the moldings carefully and rebuild them with epoxies. It was an expensive solution to a problem that could have been avoided if the wood had been checked before painting.

The first task before painting is to remove and replace any rotten wood. Gutters should be repaired and shingles replaced, if needed. Any other possible source of unwanted moisture should also be eliminated. nail holes, fill with putty and prime. Caulk the joints of the house. That includes all of the places where the siding meets the other parts of the house such as window and door frames, sills, and caps, corner boards and moldings.

The parts where the woodwork meets stone or brick steps, foundation or chimney should also be caulked. Use a high-quality latex or butyl caulk. The difference in price over oil-based products is more than made up for in the product's longer life. Caulking also makes the house more energy efficient.

Scrape loose paint to make the surface as clean and dry as possible so the paint will bond well. If the surface is powdery, (a condition known as chalking,) scrub it with a mild soap and gentle brush and rinse with a hose.

Areas such as the soffit -- the flat board under the roof overhang -- should be protected; places covered by trees and shrubbery may also need scrubbing. Dirt and grime can easily build up in such places, since they are not cleaned by the wind, rain or snow.

Mildew, a common problem here, can grow on the outside as well as the inside of a house. It looks like patches of dirt but the spots will seem to disappear if you touch them with household bleach.

Remove the mildew by scrubbing the surface with a solution of three quarts of warm water, a quart of household bleach, two-thirds of a cup of borax and half a cup of soap. Rinse the surface with cool water, let it dry thoroughly and then paint using one of the mildew-resistant paints on the market.

When the surface is thoroughly prepared you are ready to paint. Put on one coat of primer and one or two top coats of quality paint. Two coats will provide a better barrier to external moisture in a climate like ours.

Choosing between an oil-based paint and a latex paint is a matter of preference. Oil-based paints and primers are, however, a better choice for covering paint where chalking has been a problem.

You will generally want to continue using the type of paint on the house if it is performing well. It is at such times that you are very grateful if the previous owners have left you the brand name and color of the paint they used last.