He hurriedly daubs the name of his gang – the local Wee Men – a little cannabis symbol and the words "FT Polis", before adding his own personal "menshie", or name tag.
And he does all this while being videoed by a friend.
"If only other criminals were as stupid as vandals," one policer officer told the Evening Times.
"Get their pals to tape their crimes and then sign their names on the evidence - now that is daft."
The teenager behind the Parkhead vandalism was caught and the video evidence and menshies' evidence enough to convict him.
He is not alone. Over the last 2½ years police, with the help of the city council's Glasgow Community and Safety Services, have used such "menshies" and other intelligence to have more than 330 individuals charged.
That may not sound like a lot. But individual vandals can easily chalk up hundreds of offences in a single weekend.
The effect of the crackdown has been massive. The number of recorded vandalism and malicious mischief cases – to use the proper legal terms – has plummeted over the last five years.
In 2006-07 there were 17,471 in Glasgow. That fell to 12,429 in 2009-10 and 10,400 in 2010-11. That is a drop of 40% in five years.
Danny Hatfield, a police chief inspector seconded to Glasgow Community and Safety Services, said: "We are talking about dramatic reductions and that is a real positive for the community and Glasgow as a whole."
So what is behind the fall?
Well, one secret may be intelligence – and not the kind some of vandals seem to lack.
Since the summer of 2009 a team of officers now led by Chief Inspector Hatfield has been compiling dossiers on vandals, including photos of their menshies.
These are widely distributed among police and the GCSS wardens or "community enforcement officers" who patrol the city.
In turn, the wardens photograph any fresh vandalism they find, gang-related or not.
The number of such dossiers – or intel packages – has been growing. There were 71 in 2009-10 and 162 in 2010-11.
This year there have been another 51. That is 284 in total.
Chief Superintendent Bob Hamilton knows how much damage – and how many offences – a single vandal can chalk up.
He said: "We had a boy who stayed in Barlanark and who had put his tag on buildings from the East End, including the Tron Theatre, right through to Drumchapel. All in all, there was £20,000 of damage.
"There must have been 70 tags and some of them were huge. The tag was his own name, but he was affiliated with a gang.
"We got photos of the tags and community wardens in every area knew who he was and we could identify him.
"He was convicted thanks to those pictures. He got a bail condition he was not allowed to buy paint. If he was in possession of paint we could arrest him. He was only 17."
There is another secret that explains the success in the war on vandalism: the way it is cleaned up by Chief Inspector Hatfield's colleagues at GCSS.
"It used to be we cleaned up vandalism in response to public complaints to a council hotline," explained Darren Lambie, the official in charge of graffiti removal.
"Now we work on the basis of our own observations. We have to be much more pro-active."
The distinction is crucial. In the past, insiders admit, workers would motor past serious vandalism in poor areas – where there were few complaints – to scrub away less serious graffiti in better-off parts of the city, where citizens were faster to reach for the phone.
The GCSS team of graffiti clearers are bolstered by offenders doing community service.
The result? Graffiti can disappear almost as quickly as it appears, although, not, some officials acknowledge, in some hard-to-reach places, such as railway sidings.
This is crucial. Because, as officers like Chief Superintendent Bob Hamilton know, there is a clear link between vandalism and other crimes.
Chances are, the more of the former you have, the more of the latter.
"Vandalism is a key indicator," said Chief Superintendent Hamilton.
"It means there is a lack of proper control, if you like. Can folk let spray paint lie for days? That means the police are not interested or the council is not interested. And then nobody becomes interested."
In New York, they famously called this the "broken windows" principle, the idea that fixing the small problems, repairing vandalism and catching the perpetrators, would help sort out the bigger issues.
In Glasgow the link between low-level vandalism and serious violence – the so-called Group 1 crimes such as murder and serious assault – is also relatively straightforward.
The two police beats with the most reported cases of vandalism – Scotstoun East and Govanhill West – also happen to have the two highest levels of serious violence.