HAD SOCRATES been an interior decorator, rather than an instructor of Greek youth, we would today celebrate an expression something like "Know thy paint." For as Socrates well understood, there is much value in knowing the contents of anything, be it one's head, a can of soda or a pound of hamburger. Paint is no exception.

Yet several centuries after Socrates urged truth in labeling, paint remains a mystery to the average painter. Except for variations in color, it all looks pretty much the same. Authorities have tackled this problem by inventing catch phrases, such as "buy only reputable name brands," and "you get what you pay for."

Why do some paints cost $4 a gallon and others $15? Is latex paint better than oil because it doesn't smell so much? Are latex and oil paints interchangeable? What makes a gloss paint glossy and a flat paint flat?

A handful of states, including Virginia, require accurate labeling of the chemical contents of paint sold commercially. Paint companies have responded by listing ingredients almost universally. If you know how paints are put together and how various ingredients function, argues Charles Rock, assistant supervisor of Virginia's Pesticide, Paint and Hazardous Substances office, which enforces the law, you can read a paint can label and have a fair chance of knowing what you're paying for.

Paints you would use to decorate the inside of your home are divided into two basic categories -- latex and oil. The difference between these two is in the paint "vehicle." The vehicle (or liquid part of the paint -- not the pigment) is made of "volatile" (solid resins) and "non-volatile" (evaporating liquid) components. In latex paint these are synthetic plastics, such as acrylic and vinyl, and water. The vehicle in oil-based paints is made of either natural or synthetic (alkyd) vegetable oil resins (soya, linseed and cottonseed, for instance) and mineral spirits.

Because the vehicles in latex and oil and completely differnent, neither can be used on every surface. More about that later.

Pigments make up that part of the paint you actually see. Acording to their quality, they also determine how well the paint you buy will cover what you intend to cover.

The major ingredient in paint pigment is a chemical called titanium dioxide. If you look at a paint label, you will see this chemical displayed prominently, for it is, says Rock, "the backbone of the paint industry." Used with titanium, which is white, are tinting agents and, depending on the quality of the paint, various proportions of "extenders," such as calcium carbonate and silicates.

Al Wendt, the technical sales manager (former laboratory technical director) for Duron Paint in Beltsville, Md., calls titanium (in pigment) and acrylic and oil resins (in vehicles) the "major cost bearing ingredients" in paints.

Titanium, for instance, is currently selling for about $1,000 a ton, calcium carbonate for $45 to $65. The clever consumer can, to a certain degree, measure the quality of paint he is buying by comparing the proportions of titanium to extenders, in the pigment, and resins to mineral spirits or water, in the vehicle.

Cheaper paints, which do not have the same covering, nonpeeling, washability and other important qualities, can be measured by higher levels of extenders and higher percentages of water and mineral spirits. (The thin, inexpensive paint applied in apartments when tenants move out is often "calcimine," composed essentially of calcium carbonate and water.) So says Royal A. Brown, technical vice president of the National Paint and Coatings Association (NPCA) here. Such comparisons generate controversy in the $5-billion paint industry because, Brown says, "even the smallest companies might use 100 different raw materials in their paints." And each company relies on different mixing fomulae.

The average consumer may be in a muddle if he tries comparing cost and value of premium paints. In August, 1978, Consumer Reports studied alkyd and latex semi-gloss interior paints all in the high-priced category. The two alkyd paints rated highest in overall performance, Dutch Boy Satin Eggshell and Benjamin Moore's Satin Impervo, broke down this way: *02*100% Pigment*02*100% Vehicle (TABLE) Titanium(COLUMN)Extenders(COLUMN)Resins(COLUMN)Mineral(COLUMN)Spirits Moore(COLUMN)67(COLUMN)33(COLUMN)38.6(COLUMN)62.4 Dutch Boy(COLUMN)59.2(COLUMN)40.8(COLUMN)40.0(COLUMN)60.0(END TABLE)

Yet Baltimore Paint Golden Premium Enamel, rated least best of the expensive paints tested, broke down as: 57.4 percent titanium, 42.6 percent extenders; 43.4 percent resins and 56.6 percent mineral spirits.

All three paints (this comparison used white) retail for about $15. The diffenences, except for nearly 10 percent more titanium and less extender in the Moore paint, are small. But compare these percentages to those on labels of paint selling for $3- $4 and the difference become much more dramatic.

Paints are listed in order of giossiness as "flat" (least glossy), "eggshell," "satin," "semi-gloss" and "high gloss." The consumer Reports test found that among semi-gloss paints, the glossiness ranged "from as flat as some wall paints to about as shiny as a new car." Brown says the NPC is pushing for voluntary standards in the industry to label paints according to rigorous gloss-testing methods. For the moment, however, he says it is nearly impossible to know how glossy a paint will turn out without testing it first.

Latex paints, which do not achieve the glossiness of oils, are slowly taking over the paint market, Sales of latex now represent about 82 percent of the total share, Brown says, primarily because they go on easier, do not smell as much and can be cleaned up with soap and water.

Yet industry spokesmen bear out Consumer Reports' findings that latex paints do not perform as well in several ways. Alkyd paints look better after they ve dried, are less affected by water, fade less, do not peel off when you set a sticky glass on it (called "blocking") and are easier to clean.

"The oils still do the best job," says Wendt. "They are more difficult to use. They have more brush drag. But the end result is a better job."

Because oil and latex paints each have different properties and perform differently (as do glossy and flat paints) each has its own place and purpose.

Priming: A primer coating should go over any raw base, such as plaster, willboard, cement, metal or wood. You should also "spot" prime any areas where paint has peeled away (scrape the area entirely clean). Latex can be used as a primer on nearly all surfaces. On wood, however, it can bring out the grain and make an uneven surface. Latex primer cannot be used with a latex top coat on ferrous metal because the metal is left unprotected against rust.

In "high use" areas, such as kitchens, baths and some corridors, it is best to prime with a gloss or semi-gloss enamel. (Gloss is achieved by a higher ratio of resins to pigment in the paint. The resins make the paint tougher and more washable.) Oil paints are more resistant to water.

Oil paints have more hiding power. It's a good idea to choose one of these when priming wall paper or an original paint that is much darket than the paint you're planning to put down. Oil paint should not be used to prime bare wallboard. It raises the nap on the board, affecting the finish.

Finish Coats: Either latex or oil can be used for finish coatings. But remember, areas that will get wet or greasy often should be covered with an oil gloss or semi-gloss enamel. The same holds true for wood, whilch will be better protected by an oil enamel than a latex.

Follow the manufacturer's recommended covering figures on the label (usually 400-600 square feet). Flat latex paints, the most popular, are easier to apply. But this sometimes leads to their being applied too thinly, or covering more area than the manufacturer recommends.

Since some batches of the same color paint may vary in tone, paint dealers suggest buyuing a five-gallon bucket (around $2) and mixing different cans together. Do not confuse primer or finish paint with "tint base." The latter is the non-colored base used in custom-blending paints.

Next week: Rollers, brushes and other beasts. Which type to use, where and why .