A 100 metre cordon was set up around the premises at Hybris Business Park in Crossways after discovering an "extremely dangerous" gas cylinder.
Dorset Fire and Rescue Service was called to the scene shortly after 2am and firefighters began tackling the blaze with hose jets and main lines.
After a survey of the building, they were pulled out due to the presence of an acetylene cylinder and continued to tackle the flames from three aerial ladder platforms.
Incident commander and area manager Keith Bacon said: "Acetylene is extremely dangerous and for the safety of our staff we discontinued firefighting from within the building."
Members from the electricity board and local water suppliers were called in to help with the effort at the industrial unit.
Residents in the nearby area were advised to keep their windows shut throughout the day due to a high volume of smoke.
Nobody was injured by the incident and crews finally left the scene at 8pm on Saturday evening. The cause is still under investigation.