Four fire engines and 21 firefighters and officers were called to a fire at a workshop and offices in Dalston Gardens, in Stanmore.
Part of a ground floor workshop was damaged by fire.
The Brigade was called at 1345 and the fire was under control at 1506. Fire crews from Stanmore and Harrow fire stations attended the scene.
The cause of the fire is believed to be the build up of gas in the burner compartment of the spray booth.