"Don't worry, I will get you back and you won't be expecting it," the defendant said.
It was only minutes later that he followed through on his threat.
While the victim was using the backpack unit to spray a wall, Mowry went to his car where he retrieved a plastic bottle and several other ingredients, including chemicals.
Mowry threw the first improvised bomb towards his colleague.
It landed two metres away from him but did not explode.
So he did it again.
Mowry used the same method but this time, when he launched the bottle, it wedged between the backpack and the victim's back.
When it exploded, the man temporarily lost hearing in his left ear and immediately felt a burning sensation on his back.
Believing the spraying unit had caught fire, he swiftly removed it and took off his clothes.
Mowry walked towards him, laughing.
The victim asked what was in the bottle; Mowry told him it was best he did not know.
They washed the man's back but it was only later in the night when the pain increased that he sought medical attention.
Doctors found the chemicals had removed a layer of the victim's skin and in a statement to the court he said he regarded himself as lucky the outcome was no worse.
In the weeks following the incident, he found it difficult to sleep and was uncomfortable driving, the court heard.
Mowry, meanwhile, lost his job.
Counsel Meg Scally said it was never the defendant's intention to harm his friend.
He had simply wanted to "give him a fright".
Mowry said that at that time his relationship had broken down and he was abusing alcohol and cannabis.
He told Probation he was not thinking rationally when he made the bomb and had been feeling "distant".
Since the incident, he had stopped using alcohol or drugs and was back on his prescription medication, the judge said.
Mowry was ordered to pay the victim $500 and was given his first strike under the three strikes legislation.
Judge Walker declined an application to photograph the defendant, citing his mental-health issues.
