Four fire engines and around 25 firefighters were called to a fire in a spray booth on Greenford Road in Greenford.
Most of a detached spray booth next to a range of single-storey workshops was damaged by fire. There were no reports of any injuries.
The fire is believed to have been accidental and caused by a portable gas heater.
A London Fire Brigade spokesperson said: “If you’re getting an older heater out, check that it is in good condition. If it’s sparking, wires are loose, safety guards or other safety features are missing, or if it’s showing signs of damage, replace it with a new one or get it tested and repaired by a qualified electrician.
“Check that the heater isn’t on a recall list too and always buy them from a reputable seller. Fire Investigators have attended serious fires in recent years caused by non-compliant and counterfeit heaters that have been recalled due to safety concerns.
“It’s also absolutely vital that heaters are kept well away from curtains, furniture, paperwork and are never used to dry clothes. Always sit at least a metre away from the heater, particularly those using them whilst working from home, as it could set fire to your clothes or your chair and position them where they won’t be knocked over and away from pets.”
The Brigade was called at 1859 and the fire was under control by 1946. Fire crews from Southall, Ealing, Wembley and Park Royal fire stations attended the scene.