The fundraiser at the station in The Strand was held for the fourth year in a row from 10am until 3pm last Saturday.
About 30 volunteers pitched in, helping to wash a record 130 motors and raising £1,312 for the sea rescue charity.
But, for the first time in four years, crews were called out to an emergency half way through the day.
Three volunteers were chosen to respond to the call at 1.04pm, which was for a man in the water near Kingsdown.
Within minutes, they located him and his red kayak off The Butts.
He had been fishing from his kayak when it capsized. In his attempt to get back into the small boat he got a fishing hook caught in his hand.
Walmer RNLI lifeboat operations manager Denis Brophy said: "We launched the Atlantic lifeboat within six minutes and we were at the scene in a further four minutes.
"It was a very quick response time. The crew managed to pick him up and retrieve the kayak. They gave him casualty care and oxygen on the way back to the station.
"They took him into the station's special drying room to warm him up before the ambulance arrived and took over."
The man, named Will from Canterbury, was later taken to the QEQM Hospital in Margate.
He returned to the station the following day to thank the volunteers.
Mr Brophy said: "It did slow the car wash down for a few minutes but it only added to the day.
"It made people aware of why they are supporting the lifeboat and how their contributions help."
Thanks have been given to Stuart Martin and Neil Bond, who are not lifeboat volunteers but still gave their time to help on the day.
Mr Brophy said: "We're delighted to have washed three more cars than last year.
"Thank you to the general public for their support. We really appreciate it."
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