ALL OUTDOOR paint is not alike. Experts don't agree which of the two major types, oil-based or water-based paint (latex) is best for exteriors.

Latex paint doesn't require a paint thinner, is nearly odorless, is less of a fire hazard and usually cheaper than an oil-based paint. Spills can be wiped off with soap and water. Because of these reasons, says Al Wendt, director of technical sales for Duron Paints, latex paint sales are much higher than oil-based paint sales.

Oil paints can be removed only with turpentine or mineral spirits and the odor lingers for two or three days following application.

Both latex and oil come in a range of luster or shininess: flat, eggshell, semi-gloss and high gloss, though it's often hard to get a flat, nonglossy paint in oil. Most exterior oil paints contain varnish to prolong their life, explains Jim Palmer, an officer with the Northern Virginia chapter of Painting and Decorating Contractors of America (PDCA). The varnish makes for a glossy rather than a flat finish.

The National Paint and Coating Association's (NPCA) booklet on painting exteriors states, "most wood surfaces may be topcoated with either latex or oil-based paints . . . A paint with a higher gloss is preferable on exterior areas such as doors and trimwork where dirt or abrasion can be a problem."

Oil paints are best for high gloss trimwork. Flat (less glossy) paints work best on siding, masonry and stucco.

Jim Palmer of PDCA, who is also owner of Palmer Brothers Painting Contractors, says paint manufacturers are loathe to make oil paints because of the expense of linseed oil (oil paints cost about $20 a gallon while a top of the line latex paint is $15 a gallon). Although consumers like the ease of painting with latex, "I prefer a top quality oil-base paint. It penetrates a wood surface, while latex only adheres to it. Oil paints keep the wood lubricated."

Because latex paints are 55 percent water, Palmer says, the wood absorbs this moisture, the moisture evaporates, leaving the wood dehydrated. Palmer recommends oil-based paints for decks and railings, too.

Ann Shapiro of Jaffe New Ork Decorating Co., a painting contractors' firm on 911 13th St. NW, says the firm recommends oil paints for exterior and interior woodwork and latex paint for walls and ceilings.

"On surfaces that have never been painted, or areas that have become bare, the first coat should always be a prime coat," advises the NPCA booklet. A primer coat seals proous surfaces so the topcoat won't soak in and dry unevenly. It also forms a tight bond between the topcoat and underlying surface material.

"The paint you select for a topcoat will usually recommend a type of primer on the label. On new wood, many prefer an oil-based primer," says Ken Zacharias, technical representative for the NPCA.

James Clark of M.A. Bruder & Sons Inc. (MAB Paints' representative in the Washington area at 3312 Wisconsin Ave. NW) says "an oil-based primer should be used with old lksurfaces, while new surfaces do well by either an oil or water-based [latex] primer and finish," adds Clark. Personally, he says, he prefers to use an oil-base primer for both types of finish, latex or oil. "As with paint, an oil primer penetrates the surface and makes for a better bond between the finish and the primer."

When buying paint for your project, but all the paint you'll need at one time. This avoids extra trips to the store and reduces the chances of a slight color variation between batches.

The amount of paint needed depends on the absorption of the surface you're painting. It also depends on the tyupe of paint and the amount of surface area to be covered. To determine the amount, compute the surface area -- measuring the height and width of each area and multiplying to find the number of square feet.

See the paint can label for the number of square feet that a gallon of that paint will cover. Divide that into the number of square feet. This will give the number of gallons per coat to buy. Remember, warns the NPCA, rough surfaces have much more surface area than smooth, so they need more paint.

Ray Scott, president of the Northern Virginia chapter of Painting and Decorating Contractors and owner of Mister Rainbow Painting in Springfield, says a general rule of thumb is one gallon of paint per 300 square feet.

The brush you use can make it easy to do a long-lasting, professional job. The NPCA says three sizes of brushes are needed for exterior jobs: 1- to 3-inch oval or sash brushes for painting window frames and moldings; 2 1/2- to 3-inch brushes for shutters, windows and door trim; and 3 1/2- to 4-inch brushes for larger areas.

A 7- to 9-inch roller might be more efficient for large flat surfaces. "However," says the booklet, "a roller should not be used for the primer coat on wood because brushing gives better pentration and more complete coverage."

Oil paint requires a bristle brush for a smoother finish. For latex paint, use a synthetic (nylon, polyester) brush. A natural bristle brush can be ruined by a water-soluble latex paint.

A general rule is that the more expensive the brush or roller, the better.

Before you start to paint, be sure to prepare the surface you're painting. A poorly prepared surface is often the cause of later paint problems, notes the NPCA.

First thoroughly inspect the exterior for peeling paint, dirt, grease, cracks, knots, rusty nailheads, bare areas and mildew. Scrape all areas clean of peeling paint and rust. Jaffe Decorating's Ann Shapiro says sometimes it's necessary to burn off the peeling paint, although this is an expensive procedure.

"However, it's necessary if you have peeling paint," says Shapior. Burning gets to the base of your surface, allowing you to see of a leak is causing your paint to peel. Peeling paint, something that plagues homes across the whole country, is often caused by a leak or moisture trapped beneath the paint or by painting over a surface that is too hard, smooth or glossy.

Ray Scott says his company never burns off paint. "In a lot of counties there are ordinances to prohibit burning. It's particularly dangerous for an amateur painter. Something can be ignited underneath the surface and cause the whole house to burn down. We advise scraping and sanding only."

Jim Palmer, although his company does burn off paint, also says it can be dangerous for amateurs. "When we do it we have two hoses present, and someone stays on for an hour after we're done to check for smoldering. If you have a contractor burn off paint, ask for a certificate of insurance before the job gets under way," suggests Palmer. Burning is necessary in some instances, says Palmer, when there are so many layers that the substrata can't support another coat.

After scraping and/or burning, wash your surface of oil, grease and dirt with a commerical cleaner. At Bruder & Sons they recommend cleaning with tri-sodium phosphate such as Spic 'n' Span.

On siding remove dirt and chalk with a similar solution. Some white paints are designed to chalk to stay clean.

All traces of mildew should be scrubbed off with a stiff brush, and then the surface should be cleaned. The NPCA suggests the following cleaner: one quart household bleach with three quarts warm water. Rinse thoroughly with clean water before painting.

Caulk cracks in siding or open joints between building materials. Prime painted surfaces before caulking.

Sand or wire-brush away rust on iron and steel.

Use turpentine to remove resin bleeding from wood knots in siding. Seal with a special knot sealer available at paint stores.

Sand rusty nailheads, then sink them slightly below the surface with a hammer. Apply corrosion-resistant primer over the nailhead, adding putty if necessary. Then sand smooth.

Fill rough, unpainted masonry surfaces with masonry block filler -- a thick material that makes a smoother, less porous surface.

Rough up glossy painted surfaces with sandpaper -- especially under eaves or overhangs. If the surface is too smooth, the new paint will not stay on as well.

Seal plywood, because the edges of sheets absorb moisture.

Remove all rust, dirt and grease as well as any flaking paint on metal. Sand the surface. Then prime with a corrosion-resistant primer. Flaking is caused by applying the coating to a moist or excessively hot surface, using an inferior coating or applying the coating unevenly.

Hot, humid weather is not a good time to paint -- neither is extreme cold. Ideal painting temperature is 60 degrees, according to most paint labels.

When using latex paints, says the NPCA, the temperature should be above 50 degrees. If the temperature is above 70 degrees, paint in shaded areas, following the sun around the house so that the paint won't dry too quickly.

If the surface you're painting is too hot, the paint dries more quickly than it's supposed to and it doesn't stay on.

If the paint dries too fast, lap marks will show. NPCA recommends painting when the morning dew is gone and when sunlight is not direct.

Paint should be stored in temperatures above freezing and under 90 degrees, says Palmer.

Humidity won't hurt your paint, adds Clark, but it will slow down the drying process. This also can hurt the final product because if the paint is not dry by sundown or at least almost dry to the touch, dew will settle on the newly painted coat.

"We have the worst of both weather conditions here," says Palmer, "extreme heat and extreme cold." The insulation we use in the winter causes our paint jobs to peel, while the air conditioning we use in the summer does the same.

To prolong the life of a paint job, the NPCA suggests two topcoats. "The first should be applied with a brush; the second with a brush or roller."

Ann Shapiro says that Jaffe's painters use one coat if the house is in good condition, and more if it is not. "However, if we're changing the color of a house, we always use two coats," she says.

Jim Palmer says, "Two coats are vastly preferable. The cost of the second coat should only be about half as much as the first since no surface preparation is required." Palmer adds that you might want to give one side of your house an extra coat if it receives the brunt of the bad weather.

After mixing your paint, split full gallons between two containers -- it's easier and safer to use.