New Zealand art enthusiast Michael Williams was admiring works at the National Gallery of Ireland in Dublin in 2012, when a man appeared beside him and suddenly punched forward - and through - a Monet painting.
The attack caused $15 million of damage to the painting, Argenteuil Basin with a Single Sail Boat - the gallery's only work by impressionist Claude Monet - which had to be painstakingly restored over 18 months.
Mr Williams said Andrew Shannon told him he felt unwell and had heart problems and had fallen forward, but Mr Williams said that sounded rehearsed.
"He said something to me like 'I felt faint and I fell against the wall'. To me that was a prepared excuse."
Mr Williams quickly moved the man to the centre of the gallery floor so he could not do any more damage.
The jury at Dublin Circuit Criminal Court did not believe Shannon either, taking 90 minutes to find him guilty in November last year.
He has since been sentenced to six years in jail and will face a ban on going into galleries when he is released.
Mr Williams and his wife, who was with him in the gallery, have travelled back to Ireland twice to give evidence against Shannon and Mr Williams said he was extremely pleased to see him convicted.
"He's got a history of damaging works of art and paintings in England and Ireland," said Mr Williams.
"To me it's really important that this man has got a conviction for damaging paintings, so therefore he's going to be kept out of galleries around Europe."
However, despite the delicate restoration work, Mr Williams said the art work would never be the same again.
"The painting was a work of pristine condition. Though it has been restored ... the restorer explained to us that in about 20 or 30 years, that restoration work will start to show.
"It's terribly sad that an act of vandalism damages something which is of some beauty and all it's doing is sitting on the wall waiting for people to come and appreciate it."