Timothy Joseph Buzbee was found guilty last night of rape, kidnaping and robbery in the second of the Aspen Hill rape trials, a verdict that he met with an impassive stare in a hushed and sweltering Montgomery County courtroom.

The case had gone from one that was considered "far from airtight" by prosecutors to their strongest case against Buzbee with the introduction of surprise evidence late last week that Buzbee "probably" had signed his own name to a sales slip charged to the victim's stolen Exxon credit card.

Both the prosecution and the defense had been surprised by an FBI handwriting analyst's discovery last week of what he called "probably Timothy Buzbee's signature," which had been obscured by a phony name written in the same space on the sales slip.

The analyst, who had studied the documents for the prosecution, made his discovery in a review one day before testifying.

The Exxon credit card that the prosecution argued had been stolen by the rapist was found in Buzbee's desk at the surveying firm where he works after he was arrested last Nov. 5.

Montomery County Circuit Court Judge John F. McAuliffe deliberated for about two hours before finding Buzbee guilty of assaulting the 39-year-old woman on a dark residential street on March 9, 1982, robbing her and dragging her to a back yard where he raped her.

Buzbee, 25, had waived his right to a jury trial and asked that the judge determine the verdict.

Last month Buzbee was found guilty of raping and kidnaping a 15-year-old Aspen Hill girl.

He is also charged in two other cases from among the series of rapes and assaults that terrorized the upper-middle-income Aspen Hill community for nearly 18 months. He has pleaded not guilty to all of the charges.

The defense had argued that the obscured signature on the sales receipt was not that of the defendant. Defense attorney Reginald W. Bours III also suggested that the Exxon card found by police last Nov. 5 might have been planted by police or someone else in the desk, which was not used exclusively by Buzbee.

Bours also argued that mere possession of the card months after the rape occurred was not proof that Buzbee was the rapist.

McAuliffe said that after studying samples of Buzbee's signature provided by prosecutors Barry Hamilton and Bob Dean, he had determined that Buzbee signs his name "in a unique way."

He said that anyone looking at the Buzbee sample signatures and the signature on the stolen credit card slip "would have no difficulty coming to the conclusion that they were written by the same person . . . ."

The judge said he believed Buzbee wrote his own name first "without knowing what he was doing . . . and then perceived what he had done and wrote over it" with a phony name that also was signed on other sales slips charged to the stolen card.

The judge said this persuaded him that Buzbee had the card on March 14, 1982, the date on the sales slip, which was five days after the rape had occurred.

McAuliffe ruled that there was "no reasonable explanation" other than the one offered by prosecutors for Buzbee to have had the card in his possession at that time.

The judge also said he did not believe the card had been planted in Buzbee's desk by police, as Bours had suggested. "I flatly reject that" suggestion, McAuliffe said.

Bours argued that there was no reason for a rapist to risk detection by using the stolen card to buy $10 to $15 worth of gasoline.

To that, the judge said, "Not all things done by criminals follow our logic." He suggested that a criminal might use the card for the "thrill of dangerousness," and might leave it in his own unlocked desk in a "subconscious desire to be caught" or for the "thrill of flirting with danger . . . on a daily basis."

McAuliffe set July 8 for sentencing in the case. Buzbee could be sentenced to life imprisonment for the rape and a maximum of 10 years for the robbery.

Though kidnaping carries a maximum of 30 years in prison, McAuliffe indicated that he does not generally sentence defendants to a separate term for kidnaping when it is so closely related to another crime. In this case the kidnaping charge grew out of the rapist dragging the victim to the back yard to rape her.

As McAuliffe read his opinion, the rape victim sat teary-eyed, sighing occasionally, in the rear of the courtroom. After court was adjourned, she quietly expressed her sympathy for everyone involved in the case.

"My family, his Buzbee's family, everyone touched by this kind of thing," she faltered. "It does not bring happiness to anyone."

She said the experience had taught her how "vulnerable" everyone is. "A person is just going to a friend's house, getting out of a car and they're attacked. It can happen to anyone," she said. "That's what I've learned from this."