Too many now-familiar images by Chagall, Miro, Agam and Appel; too many Dalis-and dollies stuffed into mason jars-lend a nagging feeling of deja vu to most of the 150 exhibits at Wash Art '79, the fourth international show at art dealers at the D.C. Armory, through May 7.

But there is also much worth seeing, and as usual, it is in the booths of dealers who show unique objects. Many of the best are from Washington, and for the price of a Metro ticket on the New Carrollton line (plus admission to the fair) one can sample the current stock of more than a dozen widely dispersed area galleries.

Outstanding, as usual, is the joint Ewing, Sander, Lunn Booth, which is showing a fine array of photographs by the young and the famous. Most of the new talent comes from Kathleen Ewing, formerly a private, at-home dealer who has just opened a new gallery under the Whitehurst Freeway, at 3020 K St. NW.

Middendorf/Lane is giving exposure at the fair to Washington newcomer Robin Rose, along with better known artists Joe White, Sam Gilliam and Leon Berkowitz, and really old masters such as Thomas Hart Benton and Stuart Davis. Barbara Fiedler has a handsome booth featuring Dorothy Preston, Carroll Sockwell, John Cunningham and Vint Lawrence, while Jack Rasmussen has concentrated on the dramatic paintings of Reginald Pollock, including a striking new triptych.

The little-known Gallery Amerasia (based at 2142 F St., NW)-a 15-artist co-op of Asian painters, printmakers and photographers who live in the area - is showing some charming watercolors in the traditional Chinese mode by Hsien-Ming-Yang. The woodcut portrait of Abraham Lincoln surrounded by the Gettysburg Address written in Chinese Characters is surely the most unusual piece at the fair.

Ramon Osuna is showing his worst side in a display of paintings by John Stewart, all femal e rumps tightly wrapped in satin. A badly lit sculpture by Emillie Benes Brzezinski sets incongrously nearby.

Art fairs are designed chiefly to give print publishers and dealers working in mass markets a chance to supply each other with stock. (The public, however, is encouraged to browse and buy, and in some cases to haggle.)

Among the print publishers are some of the finest. Petersburg Press is featuring several images by David Hockney, among others, while Jacobson of London is showing beautiful color screenprints by William Tillyer. Sidney Janis Gallery Editions is showing three-dimensional multiples by George Segal and Tom Wesselman, along with the Brooke Alexander, Landfall Press and Maeght Editeur all are worth a visit.

Among out-of-town dealers, Jacques Kaplan from New York has come to be counted upon to provide something first rate, and this year's entry, cunningly entitled "The Museum Island", keeps up the standard with seven important works, including a large black wall by Louise Nevelson, several paintings on paper by Richard Lindner, and a major painting by Larry Rivers called "Dutch Master I."

Other out-of-town dealers of note are Judith Posner of Milwaukee and Dolly Fiterman of Minneapolis. Fiterman is showing several prints by the inimitable William Crutchfield. Spectrum Gallery of Vienna, Austria, has several images by Hunterwasser and decorative prints by a Yugoslav named Rabuzinov. Among the many tapestry exhibits, Muhelyart of Switzerland's is the most impressive, notably for one work based on a painting by Paul Klee.

The art fair always turns up some new wrinkle in the business of art and its satellite businesses. Just inside the entrance to the exhibit hall is a man touting "The Collectors Card," a new credit card that will advance a purchase up to 40 percent of his gross annual income to buy art - with interest, of course. The card alone costs $50 per year.

"Does anyone actually buy these?" the man was asked.

"Would we be here if they didn't?" he replied.

The fair continues through Monday, May 7. Hours are noon until 9 p.m. every day. Tickets are $3; children under 12 are admitted free. A helpful catalogue is available for $3. CAPTION: Illustration, no caption.