Air carrier maintenance personnel reported personnel were improperly cleaning an unknown chemical substance that was later discovered as hazardous.

Narrative

While initially investigating a pilot write-up; after their walkaround on a routine turn; involving a missing brake wear pin indicator for the aircraft's #4 main landing gear brake; it was observed and discovered that an unknown substance was covering the RH inboard engine cowl. The unknown substance at the time was clear and oily; while smelling of a combination between peanuts and burnt tires. It was quickly determined that the source of the substance was originating from drain holes in the forward cargo compartment. After opening up the forward cargo compartment door; a puddle of the substance was found on the floor panels where a barrel of it had been previously stored on the inbound flight and clearly had been punctured; leaking said substance throughout the forward cargo compartment. Maintenance personnel were then directed by our on-shift Operations Manager (OM); to start cleaning up the substance per work order; forward cargo compartment spill; before the chemical had been properly identified and steps were taken to properly mitigate any safety risks in dealing with the substance. It was only then observed mid-way through the start of the cleanup that the substance is a skin irritant and not recommended to come in contact with skin as well as not recommended to be inhaled. Gloves were worn by maintenance personnel during the initial clean up; but not upon first discovery of the substance; and no respirators were worn during any of the event.Suggestion: Upon discovery and in general; when dealing with unknown and potentially hazardous chemical substances; proper safety precautions and procedures should always be taken to mitigate any health risk and reduce exposure for maintenance personnel. It should also be evident to personnel in management positions that maintenance actions should not be directed until proper identification of the substance has been completed.