Police have issued CCTV footage of the suspect as part of their appeal.
The victim, 19, was with a friend when a man threw a corrosive substance at them from a cup.
The attack took place in Thornton Road in Thornton Heath, Croydon, on December 7 at around 8am, leaving the teenager blinded and scarred.
His friend, 25, required hospital treatment.
There was no build up to the incident and the victims were entirely innocent, officers have said.
Detectives have now released video footage of the suspect punching the victim in a clash three months before the acid attack.
The CCTV from September 7 shows the victim being hit twice by an attacker with a bicycle chain wrapped around his knuckles.
The attacker first knocked at the teenager’s door saying he had a letter for him.
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The CCTV then captures him throwing punches at the victim.
Crimestoppers have put up a £5,000 reward in the hunt to catch the acid attacker.
Alex Loukas, London Regional Manager at Crimestoppers, said: “This horrific incident has left one teenager with life-changing injuries, both victims traumatised and has shocked the community of Thornton Heath in Croydon.
“The individual behind the acid attack could strike again, so it’s critical that anyone who knows or has suspicions about who was involved contacts our charity 100% anonymously.
“We need the public’s help to bring those involved to justice and help the victim to recover and the community to feel a dangerous individual is no longer a threat.”
The suspect is described as a white/Asian male, slim build with a black hooded jacket, and dark coloured trousers and shoes/boots.
On each occasion he was wearing a light blue medical mask and a dark coloured baseball cap.
