THE CURRENT art boom has changed the Washington market in countless ways, inflaming interest, driving up prices and turning casual investors into rabid collectors. It has now spawned a new kind of entrepreneur: the know-nothing dealer.

"I tell people right up front that I don't know anything about art," says David Harrison, who after selling 18th and 19th-century American and European paintings out of the garrage of his Potomac home for a year has now opened "Antique Art Galleries" in Kensington.

A better name might be "The Caveat Emptorium."

"It's a hobby that went wild," says Harrison, 45, whose small space at the rear of a courtyard over a plant shop is jammed, frame to frame, with more than 15 paintings and watercolors attributed to old masters like Albert Bierstadt, Jasper Cropsey, Thomas Moran, Frederick Church and a host of other lesser-known Americans and Europeans.

Many of the Washington area's 300 antique and flea-market dealers are taking advantage of the overheated market in early American painting, where demand is now outstripping supply. But Harrison's operation is unique. His gallery deals exclusively in pictures, and chiefly in the kinds of second- and third-rank 19th-century American paintings now being hauled out of attics, basements and antique shops to satisfy the hungry hoardes. The going prices of these often unauthenticated works, as well as the quality, is often lower than most well-established art galleries care to bother with.

"Every antique shop has some paintings they've picked up over the years," Harrison says, "and if you want to buy a piece of 'antique art,' [by which he means pre-20th-century paintings] for under $1,000, where else is there to go but to them, or -- if you want a bigger selection and discount prices -- to me?"

Harrison is also a salesman for an area medical lab and sells real estate on the side. He hopes to chuck it all if his gallery at 3760 Howard Ave. takes off. And it could: "Business has been fabulous," he says.

That's the case all over town, as antique dealers cash in on the painting boom. "It's true," says Edmund O. Carl Jr., the heir of "Call Carl" car-repair service, who now runs Carl's Antiques in Olney. "I used to have quite a few paintings, but they're the hottest items in the antique business right now." Maurice Silverman, of Silverman Antiques in Alexandria agrees: "We're heavily into paintings."

"It's a reflection of the market," says William Truettner, curator of 18th and 19th-century art at the National Collection of Fine Arts. "People are willing to buy anything these days as investments and inflation hedges, including some terrible paintings."

There is no way to measure the dollar-volume of the "antique art" boom. But since antique shops here outnumber art galleries by more than two to one, it clearly constitutes a hefty chunk of the market. Harrison, almost by accident, has established his own corner of that market, and the self-proclaimed "discount antique art dealer" could be the first of a new breed.

Harrison says all the works in his gallery are on consignment. He himself owns only a few small watercolors: "I don't have the money to buy things, but I have nine or 10 people who regularly consign things to the gallery." Many of them are part-time art brokers, others scrounge flea markets, estate sales and auctions. A few are "pickers" who go from town to town buying and selling. The consigner agrees to a fixed price if the painting is sold, and Harrison's profit is whatever he can get on top of that, usually about 10 percent.

Although Harrison's biggest sale ever was a painting by Benjamin West that finally went for $32,000 -- down from the original asking price of $80,000 -- prices generally range from $100 to $1,000. The average sale recently has been in the $300-to-$500 range. Birth of a Notion

Harrison's plunge into the art business began a few years back by accident, when his wife's uncle, a patron and collector in France, left the couple "five or six drawings and two or three oils." What he did with the inheritance is as unusual as everything else about Harrison's dealership. "We ran a garage sale ad in The Post from our house in January 1979 and were flooded with people who wanted to buy. But we were also flooded with people who had something to sell, and wanted someone to do it for them.

"So we worked out this setup: "I'd take the paintings on consignment, and in lieu of knowledge I offered a 100-percent money-back guarantee. For one week after sale was made, buyers could bring back the art and get a complete refund. If they wanted to extend the trial period to two or three weeks, they could do that, too."

It worked. During the first years, Harrison sold 740 paintings to 178 people, about half of them decorators and other dealers from New York. His total profit $5,200. "That's not bad for a garage sale," says Harrison. "I should make $12,000 this year, if things keep up the way they're going."

With ads in Antiques Magazine, The Washington Post and the New York Times, "I've developed a reputation for moving things fast," he says. "We haven't returned anything to a consignor unsold for over a year. Some things were sold for only a $5 profit, but I'd rather do that than disappoint a consignor. After all, without them, my business would just stop since I have neither the knowledge nor the capital to supply myself with art." No Gurantees

Harrison's ignorance about art is no secret. In fact, he proclaims it in large print at the bottom of everything, including lists of available works and bills of sale. The caveat reads as follows: "Authentications and attributions have been solely provided by our suppliers. None of the employees of Antique Art Galleries has independent knowledge of the provenance of works sold of their authenticity. The proprietor cannot be responsible for the accuracy of authentications or attributions, and we urge that all art be independently examined and/or appraised."

It's a very different sort of agreement from those customarily made with fine-art dealerships. Harry Lunn's customers, for example, are assured as follows in his latest cataloque of offerings: "The authenticity of all works in this catalogue is guaranteed." "It goes without saying," says Lunn.

But it does not go without saying in the auction or antique business, which is where Harrison says he belongs. "People keep comparing me with high-class art dealers like Adam Davidson.Their business is super-expensive paintings, and so are Montrose and Liros galleries," he says. "I'm competing more with Silverman's Antiques in Alexandria, Walter Reed Antiques in Bethesda, Carl's Antiques in Olney and Thieves' Market on Route 1."

Harrison freely admits that 10 percent of what he sells comes back within a week. "But only 10 or 15 paintings came back as misrepresentations," he insists. "The rest were from husbands who bought without their wives, or vice versa." And what if more than the one trial week has passed? Harrison says it only happened once, "and I gave them their refund." p

One couple took a painting to an appraiser downtown who, in turn, called in the Maryland state's attorney's office, suspicious that the painting was not what it appeared to be. After observing the gallery closely for four months and asking innumberable questions, the state's attorney has just written Harrison to say, "After a thorough and extensive in investigation, we could find no impropriety on your part or any of the persons employed by Antique Art Gallery." It would be interesting to know what proportion of art businesses could survive such an inquiry. Fast and Confidential

Asked why anyone with good paintings would want to deal through him rather than through a more established and knowledgeable dealership, or through an auction house, Harrison says, "I represent consignors who are in a rush for their money, and we sell fast."

"Of course this means the public has to be wary," says one client, "but as a consignor, you can set your own terms. If you take it to an auction, you'll have trouble putting on a high reserve, and if the piece isn't sold, you'll still have to pay the auction house their commission. This is also faster than waiting for the next auction. Harrison frequently turns things around in a few weeks. Also, the fact that he was willing to use paintings I didn't want as collateral for something I did want makes a big difference to someone like me, who does this on a small scale and on weekends."

Harrison says that a new collection that has just come into his gallery includes works by Bierstadt, Cropsey, Church, Chase, Twachtman and several others."The consignor called and said he had a family collection to sell, and heard I'd sell them fast and confidentially. I felt like I'd arrived!" That crop of paintings -- priced from $4,500 (for a painting said to be by George Fuller) up to 20,000 (for one attributed to Albert Bierstadt) -- is now on view in the gallery.

The question of where Harrison gets his paintings, and whether he might be unwittingly used as a fence for stolen goods, is bound to cross the mind of anyone who talks to him. "I'm protected by the people I deal with," he says. "People have come in off the street and offered me things, but I don't take any chances. My consignors are auction and flea-market nuts who want their money fast so they can go to the next auction. They're looking for the big hit."

Among them are a judge of the D.C. Superior Court, well-known in art circles; a famous tennis pro; the wife of a wealthy area builder; a lawyer; and a musician/geologist. All speak well of Harrison, and some fondly.

The judge was particularly happy about a Bricher watercolor he'd bought from Harrison for $1,400: "I have no reason to doubt Harrison's integrity, though I have no overwhelming respect for his knowledge. He sells what comes to him, every once in a while you hit a bonanza."

Geologist-musician-writer Robert Hazen knows that Harrison's operation "has raised a great deal of controversy," but says that "even if you're buying from the most expensive gallery in the world, you should still get a second opinion. It costs $30 to have a half-hour consultation with Ted Cooper of Adams Davison Gallery, but it's worth it."

Russell Burke, vice president of Sloan's Auctioneers here, will give a verbal appraisal without charge. So will William Truettner at the NCFA, but he will give no monetary advice, and appointments must be made well in advance.

Harrison says that a big problem comes from other dealers, many of whom flocked to his Potomac garage when his newspaper ads first appeared. Some of them, Harrison says, have been bad-mouthing him ever since. One Bethesda dealer felt compelled to hang up a letter in her gallery announcing that she has no connection with Antique Art Galleries. Says Ted Cooper of Adams Davison: "For months we've had people coming in here with paintings from his gallery and 80 percent have been grossly over-painted by a restorer, or misattributed -- not only to the wrong artist, but to the wrong century!"

In defense, Harrison says Cooper's 80-percent estimate is "ridiculously high." He concedes, however, that "last year we had about 15 paintings that weren't what the consignors said they were, and we sold them wrong. Several were taken to Mr. Cooper, and because the first two or three from us were wrong, he assumed everything was bad. He doesn't remember the 100 that were right. The bad ones came back like bullets.

"Now I advise my customers not to tell appraisers where they got the work. I don't want the paintings to suffer because of their feeling about my business. It takes a long time to build a clientele, especially if you suffer the harassment of the established dealers. But you have to build your own clientele. It will probably take me five years."

To accommodate his clientele, Harrison made some changes in his pricing policies last week. "We used to have prices on the wall and sell at about half that price -- because people like to dicker so much. But a number of people who hadn't been here before were put off by that, and so we've decided to put the actual prices on the wall. The paintings are now priced as shown." t

Though many will remain skeptical, Harrison says he's tired of impertinent questions. "People are always asking," "How dare you sell paintings? How dare you be in our business?' Well, everybody has to start somewhere."