Sears Roebuck & Co. plans to open five or six paint and hardware specialty stores in the Washington area this year as part of the first full-scale expansion of a new store concept Sears has been testing for three years.

The nation's largest retailer says it will open 50 such stores this year, in hopes of holding on to business that is being strongly courted by do-it-yourself home-center chains.

Beyond 1986, Sears plans to open about 50 more stores a year, according to the trade publication National Home Center News, which calculates that Sears is the largest home-improvement chain in the country, selling about $4 billion of home-improvement goods a year.

The Sears Paint & Hardware stores will be small -- 4,000 square feet -- and sell a wide variety of hardware, automotive products, lawn and garden items. Goods will include ladders, portable electric tools, window and door hardware (including keys, which many traditional Sears stores do not sell), lawn mowers, car batteries and filters, weater heaters, garbage disposals, charcoal grills, electrical wiring and plumbing items. Also included will be all lines of paint now sold at traditional Sears stores.

However, the selection in each line will be much more limited than a traditional Sears store, or a hardware store for that matter, industry officials note. Unlike other hardware stores that sell three or four different brands, the Paint & Hardware stores will sell only Sears' products, including its well-known Craftsman tools. But even Sears' goods will be limited. "There will not be the depth of inventory" as in a typical Sears store, a company spokesman said. The small stores "will sell a half dozen different hammers instead of the 30 to 40 you find in a regular store," the spokesman added.

Sears has been testing the paint-and-hardware-center concept for the past three years, researching its viability in 12 stores in Chicago, New York, New Jersey and California.

Sears officials are negotiating for sites here as well as in Boston, Atlanta, New Orleans and Denver. The company also plans to add more stores to the nine already in Chicago. The Paint & Hardware stores will be situated in secondary shopping centers -- not in large malls like most traditional Sears stores.

The stores represent "a good opportunity" for Sears to expand in areas "where we're not conveniently located," said Richard Thompson, assistant director of planning for the Sears merchandise group. Thompson said the hope is to lure loyal Sears customers who otherwise would have gone to a more convenient paint or hardware store.

"We sell one out of every three gallons of paint sold in the country today," Thompson said. However, he noted it would be hard to keep that dominance with the current proliferation of paint stores. "Would you really drive by five other people who sell paint" to buy paint from Sears? Thompson asked. "We don't want them to drive to five other outlets before they get to ours."

The move toward specialty paint and hardware stores comes as the entire retailing industry is moving toward increased specialization -- opening smaller stores that carry a limited line of goods in more depth than the traditional department stores.

For Sears, these store openings come as the company also is opening small department stores -- from one-sixth to one-third the size of the traditional Sears store -- in rural areas that large retailers long have ignored in favor of the more populated and profitable urban areas.

The paint and hardware stores and the small department stores represent an effort by Sears to expand its market at a time when there is little opportunity for growth in urban areas, where far fewer malls are being built.

Expanding business is critical for Sears, whose merchandise business last year was extremely sluggish. Sales for 1985 were flat at about $26.6 billion. Profits, on the other hand, were down to $765.7 million from $905.2 million in 1984.

But industry officials say the Paint & Hardware stores will add relatively little to the company's bottom line. "A traditional hardware store does an average $250,000 to $500,000 of business a year," noted Wyatt Kash, editor of National Home Center News. Using those figures, the Sears specialty stores would add about $31 million to $26.6 billion of revenue collected by Sears' merchandise group last year.

Nonetheless, financial analysts are praising the concept. "This rollout is long overdue," said John S. Landschulz, an analyst with a Chicago-based financial investment firm, Mesirow & Co. "Their day-to-day business is paint and the extras, and they have been losing it to the extremely competitive home centers. This the paint-and-hardware centers is the future Sears has. No matter how good you are, you have to be located conveniently to the customer. It is necessary if they are going to keep market share."

Sears' Thompson said the company selected Washington as one of its first sites because "Sears has a good reputation in that market. It's a good market for us."

Landschulz noted, however, that Sears will face heavy competition in the Washington area, where it will vie not only with local paint stores, but also with the dominant do-it-yourself-center chain, Hechinger Co.