Sometimes, Abraham Johnson gets frustrated. Johnson, a dentist in Southeast Washington, advises patients to have their children's teeth sealed to prevent cavities. But few of his patients heed his advice.

"My patients can't afford it," he said. "It's very discouraging."

Even though there are benefits, sealants, which cost between $7 and $25 per tooth, remain underused, dentists say. Recently, however, several national dental groups have launched major campaigns to encourage their use. There are indications that their efforts may be paying off.

Developed in the 1960s and '70s, dental sealants work by forming a thin, clear plastic coating on tooth surfaces that protect against decay by creating a barrier between the tooth and food particles or bacteria.

Sealants are used primarily to protect molars and premolars in children. Unlike incisors and canines, which have flat chewing surfaces, molars and premolars have numerous pits and fissures that increase the surface area for grinding food. But they also provide places where food and bacteria can be trapped and cause cavities.

Dental sealants are best applied to children when their permanent molars erupt, usually at ages 6 to 8 and 12, according to Louis Ripa, chairman of children's dentistry at the State University of New York's School of Dentistry at Stony Brook. Most dentists and dental groups recommend sealing teeth only through age 17. But, Ripa says -- given the increasing number of cavities dentists see in older teens and young adults -- perhaps they would benefit from sealants, too. By the time they reach their mid-twenties, most people have already had most of the cavities that sealants are designed to prevent.

Children are more likely to get cavities, Ripa said, because they eat more sugar than most adults. Newly erupted molars, where 80 percent of cavities occur, are more susceptible to decay, which develops faster than in other teeth.

According to John Bogert, executive director of the American Academy of Pediatric Dentistry, sealants are relatively safe and easy to apply. They might taste bad at first, but patients would feel little except the brush applicator and a little liquid. The whole procedure usually takes seven to 10 minutes per tooth.

Ripa said that sealants can reduce decay by up to 99 percent for the first two years after application and up to 62 percent four years later. How long and how well they work depends on the skill of the dentist -- or in some states, the hygienist -- who applies them, as well as whether the teeth are re-treated when the sealant begins to deteriorate.

Nearly every dental and public health organization has endorsed sealants. The American Dental Association, for example, first approved a commercial sealant in 1972 and then accepted sealants in general in 1976. A 1983 National Institutes of Health consensus conference called sealants a safeand "highly effective" means of preventing cavities. And former Surgeon General C. Everett Koop said their use would reduce the future need for dental care.

Nevertheless, a nationwide survey conducted in 1987 found only 7.6 percent of nearly 40,000 schoolchildren examined had one or more teeth sealed. "We were surprised by the findings," said Preston Littleton Jr., deputy director of the National Institute of Dental Research, which sponsored the survey.

If sealants are safe and effective, why have so few children had their teeth sealed? "Dentistry is a conservative profession," Bogert said. "Acceptance of any new technology in medicine is often a slow process." The dental profession, in fact, has promoted sealants only since the 1983 NIH consensus conference.

Furthermore, many older dentists who were not trained to use sealants have been slow to accept them. Some fear that if a small, undetected cavity is sealed into a tooth, it could continue to grow. Others remember the ineffectiveness of early sealants. "I have a funny feeling about sealants," said Dewitt Fortenberry, a Washington dentist who does not use them. "It may be a good procedure, but it has not been perfected yet."

Actually, Ripa said, any decay inadvertently sealed into a tooth will get no worse as long as the sealant remains in place. The reason: cavity-causing bacteria would be sealed off from the food source, thus halting the cavity's progress. If the sealant deteriorated, the cavity might grow, but no more so than if the sealant had never been applied.

The key to to long-lasting sealants, Ripa said, is proper application. He advises dentists to carefully etch the teeth adequately so that the sealant bonds properly to the surface, to keep the enamel contamination-free and the tooth completely dry until the sealant has hardened.

Price remains a major impediment, dentists say. Most insurance policies do not cover the cost of sealants, and some insurers say they believe the money is better spent on preventive dentistry in the form of regular check-ups.

Some insurance companies say they are dubious about the necessity of widespread use of sealants. David Wesley, director of Aetna's dental health programs, cites recent studies showing 37 percent of American children are cavity-free. In addition, he said, people who live in areas with fluoridated water may not need sealants.

But fluoride treatments, even applied topically, cannot completely stop cavities, Ripa said. Fluorides work best on flat-surface teeth, he added, not on molars and premolars, because bacteria tend to become trapped in their pits and fissures. On the other hand, combining fluorides and sealants gives dentists "the most effective preventive tools" available, Bogert said. "Together, they can virtually eliminate cavities."

Although dentists acknowledge that fewer children are getting cavities than did children 20 years ago, some dentists regard sealants as added protection. "We cannot predict who will get cavities and who will not," said Michael Roberts, former deputy clinical director of the National Institute of Dental Research. "I would rather err on the side of preventing decay. Besides, sealants can't hurt."

The real question, said Roberts, now assistant professor of dentistry at the University of North Carolina, is whether all children should have all their teeth sealed. Roberts and others advocate sealing all molars and premolars in all children.

But many dentists say that wholesale, indiscriminate use, while harmless, is probably unnecessary. "Sealants are underused by the profession as a whole and overused in some offices," said Selig Chester, a Northwest Washington dentist.

Stony Brook's Ripa agrees. If pits and fissures are not deep and narrow and do not catch the hooked tip of the dentist's metal instrument, he said, they might best be left unsealed.

"It calls for the dentist's best professional judgment," Ripa said. "The profession may one day agree that all teeth should be sealed, but that is not the current consensus among dentists."

There are some signs that the use of sealants is increasing. According to various published surveys, about 70 percent of all dentists and 93 percent of pediatric dentists say they use sealants on at least some of their patients.Jeffrey P. Cohn is a freelance writer in Washington.