A $42,000 fine, the largest ever assessed in a Fairfax County obscenity case, was imposed yesterday on a Richmond Highway adult book store charged with 42 counts of renting obscene films.

Fairfax General District Court Judge J. Conrad Waters Jr. convicted the firm and imposed the fine after viewing 42 films depicting various sexual acts. He said each of the films, with titles such as "Landlady Lust" and "Sweet Alice the Cook," was, in his opinion, obscene.

The store, Croatan Books, 9653 Richmond Hwy., has been convicted of 75 to 100 obscenity counts in the five years it has existed, according to prosecutor Rodney G. Leffler.

Croatan attorney Richard Stanley said Waters' verdict will be appealed.

One of the prosecution witnesses was Lynn Webb, 51, a professional artist, part-time tennis instructor and chairwoman of the Mount Vernon Civic Association's community standards committee. The association has waged a prolonged campaign to force the store from the area.

"Those films are shameful," said Webb from the witness stand. "I work with young athletes, and to depict cheerleaders and football players doing things like that is absolutely unbelievable."

Defense attorney Robert E. Zuckerman argued that Webb failed to quality as an expert on how the Mount Vernon community feels about the store.

To convict a corporation under Virginia's obscenity law, prosecutors must prove that material violates a particular community's standards, using guidelines set by the U.S. Supreme Court. In this case, the films had to be found patently offensive, expressing a shameful or morbid interest in sexual excitement.

Judge Waters emphasized, in imposing the maximum $1,000 fine on each count, that "this is not a forum to get rid of this store. By no means should it be taken that I want it out of business."

Zuckerman said the store has paid all previous fines levied against it and will pay the $42,000 only if all legal appeals fail.

The films ruled obscene yesterday were confiscated in raids on Jan. 24 and March 8, 1979, according to Fairfax police investigator Rick Daniele, who said he, other officers and a special magistrate spent two days viewing the films in 25-cent peep shows in the store's viewing stalls.