Despicable Jonathan Steel,51, helped to decorate the vulnerable female’s flat in Rutherglen, South Lanarkshire, in October 2017.
But after finishing the job, the High Court in Glasgow heard last month how Steel returned to his 28-year-old victim’s property and subjected her to a terrifying sexual assault.
He was brought to justice after detectives learned about Steel’s activities.
On Tuesday, at the High Court in Edinburgh, judge Lord Clark told Steel he had no other option but to send him to prison.
Passing sentence, Lord Clark said: “Your conduct makes for disturbing reading. It’s a very serious crime. It is made even worse by the fact that you targeted a woman who was clearly vulnerable.
“It’s clear that your conduct has had a deep and lasting impact on the woman.”
Steel, of Calton, Glasgow, pleaded guilty to raping the woman, who cannot be named for legal reasons, at a hearing at the city’s high court last month.
Sentence had been deferred to Edinburgh for the court to obtain reports about the sex offender’s character.
At proceedings last month, prosecutor Liam Ewing told the court how the woman had learning difficulties and that she knew her attacker as John.
Mr Ewing said that the accused turned up at her house at 6.30pm on October 30 last year and walked in after his victim answered the door.
The court heard how the woman was in her pyjamas. She walked to her bedroom and Steel followed her there.
Mr Ewing added: “The woman said she felt scared. Mr Steel touched her on the breast and she did not say anything because she was too frightened.”
The court heard that Steel then undressed himself and he raped the female while she repeatedly told him to stop.
Steel then dressed himself and left. The woman texted her support worker saying: “I’m so frightened. Don’t want to stay in this flat any more.”
On Tuesday, defence advocate Geoff Forbes told the court that his client was sorry for what he had done.
Mr Forbes said his client wanted to participate in a prison scheme designed to rehabilitate sex offenders.
Mr Forbes added: “He accepts what he did was wrong. He has expressed his regret and remorse to me. He has expressed a feeling of empathy towards the complainer to me.”
Lord Clark also placed Steel on the Sex Offenders Register for life.