Gerrard Carr left his victim so scared in March this year that she leapt out of a first floor window to escape.
A judge heard how the 26-year-old woman thought her leg had been broken after she was struck with a baseball bat.
Carr, 36, was convicted of the attack - which included the victim being indecently assaulted - following a trial at the High Court in Glasgow.
Sentence was deferred until next month.
Carr struck the woman with a baseball bat and punched and kicked her, at a flat in Bonhill, Alexandria, on March 1 this year.
He told the jury that he "regretted" what happened and insisted that when he first picked up the bat, he had just wanted the woman to leave the house.
Prosecutor David Young QC asked: "Why hit her?" Carr replied: "I was angry - by that time my temper had just broken. I felt hurt because she had spat in my sandwiches.
"I hit her with the baseball bat, but it was a reaction. I thought that I was entitled to ask her to leave."
The court heard the victim tried to get out after being struck and that there was later so much blood, the woman was virtually unrecognisable.
She was said to have fled upstairs and was left "cowering" in fear.
Mr Young put to Carr that he "pursued" her to "carry on the attack".
But, Carr replied: "No, it was just to ask her why she had spat in my pieces."
Carr admitted in his evidence that he had attacked the woman, but denied there was any sexual element to the incident.