"It's been covered up over the years," he said.
The wall, covered in fibrolite, had received some knocks and cracks.
"All I wanted to do was pretty the wall," he said.
"Will said, why don't I take it back [to original]."
Mr Adamson said the sign has had a lot of attention, with people coming over to photograph it.
"We're limited to what the heck we can do with it."
He said his grandfather, James Adamson, built the service station about 1927, "certainly before 1930".
The family lived on the premises with three children.
The workshops have been added on since.
The advertisement still holds true, as the station does stock Firestone tyres.
Mr Bonne said they can see traces of an earlier sign for Castrol fuel underneath the Firestone sign.
Photos displayed at the station, dated 1942, show a different Castrol sign on cladding, presumably covering the Firestone sign.
"For that to have been weathered like that, it must been done a number of years since 1942."
The Sign Factory has suggested taking a hi-resolution image of the sign and displaying it full-size on the wall.
But that's an expensive idea, said Mr Bonne.
He has contacted Firestone in Masterton to see if they are interested in the sign.
Firestone Masterton manager Russell Hodson said he had forwarded information about the old sign to their head office and had yet to hear back.
As an "old Firestone man from way back" he was intrigued by the discovery but said the branding had moved on "quite significantly".
