Fairfax County, the Washington jurisdiction that spends more money enticing companies to its borders than the other localities combined, yesterday stepped up its how-to-sell-the-county campaign.

Fairfax started by flyings six trade-press journalists to the county, footing their hotel bills, wining and dinning and busing them around the county's better sights for a day. Part of the bus tour centered around Reston, included pointing out Redskins quarterback Joe Thiesmann's house.

The second step: They poured on the sales pitch as thick as the New York strip steak served for lunch and as sugary as the cheesecake dessert, with testimonials from four local business people, a slide presentation and new county growth figures.

Step three: They established a news peg, the fact that the Sperry Univac computer manufacturing firm is building a $12 million, 27.5-acre business complex near McLean in Fairfax county.

County officials brushed off suggestions that the county's phenomenal growth could slow, as is expected in other parts of the country, because of increasingly higher loan interest rates.

"We have seen no slowdown in activity," said James C. Cleveland of the Reston Land Corp. "We don't see any slowdown on the business side.I think there will be some slowdown on the residential side. We expect no complete shutdown like we saw in 1973 or 1974."

"I think people do feel that this is only a temporary thing," said David A. Edwards, executive director of the county's economic development authority. "Maybe people will go a little slower."

The promotion continuing today with a business exposition at Springfield Mall is part of the county's campaign to expand its tax base with commercial and light industrial development. that theme has been picked up by other jurisdictions lately but not to the extent that Fairfax has done. The Fairfax County Economic Development Authority has an annual budget of $743,000. More than $300,000 of that is just for advertising.

Ted Levine the county's New York public relations expert, said yesterday that the promotions such as the one yesterday and another in New York last spring are effective in publicizing the county without the expense of paid advertising.

Last May, county and state officials trekked to New York and sponsored a luncheon at a Manhattan hotel for about 50 reporters, explaning the county's features. Levine said about 12 articles about the county were published as a result.

Edwards, however, said he was unclear what tangible effects the New York trip had on business people. Edwards said the program was successful "just to have these people aware that there is a thing such as Fairfax County."

The tour yesterday spanned the county's positive sections from the concrete sterility of parts of Reston to the money, southern hospitality of the A. Smith Bowman distillery where Virginia Gentleman bourbon is made. To point up the county's cultural amenities, reporters also toured Wolf Trap Farm Park.

The announcement of the Sperry-Univac expansion was made at a reception in the Westpark-Westgate business development complex near Tyson's Corner. A tour guide even boasted that I. Magnin and Neiman-Marcus stores were locating in Tyson's Corner, a motion dismissed later by Edwards as "real estate rumblings."

During the promotion it was disclosed that this year the county has 1,413 companies located there, a 24 percent increase in the last two years and a 180 percent increase since 1970. About 57,000 people are employed in the county, a 33 percent increase in the past two years and a 131 percent increase since 1970. Edwards said the county has 17.6 million square feet of office space in use, a 22 percent increase in the past two years and a 96 percent rise since 1970.