Isocyanate clearcoat in a borrowed bay
The main spray booth is out of service for two days waiting on a fan motor, and a regular customer's 2018 dual-cab ute is booked for a clearcoat blow-in on the left rear quarter and rear door. The only free space is the general workshop bay, which has a roller door, a dust extractor rated for grinding dust, and no filtered supply air. The painter has a half-face respirator with A2P3 cartridges and plans to mix 500 mL of a 4:1 acrylic urethane clear with 10% thinner and spray it with a conventional gun at 31°C. Two other technicians are grinding a guard about six metres away, the roller door is half open, and the customer has been told the vehicle will be ready tomorrow. The painter argues that the area is small and the job will take 'ten minutes of spraying'.
1. List the hazards in this job and explain, using the hierarchy of controls, what must happen before any product is mixed.
Read the worked answer
Hazards, in order of severity
- Isocyanate aerosol from the 2K clear. Sensitisation and occupational asthma can follow a single short overexposure; there is no reliable odour warning, and no cartridge filter is approved for isocyanate spray.
- Solvent vapour and fire risk. A conventional gun in a bay with a half-open door gives no controlled airflow, and a dust extractor motor is not rated for a flammable atmosphere.
- Dust from the grinding six metres away. It will settle in the wet clear as nibs and dull patches, and it is an airborne irritant for everyone in the bay.
- Other people. The two technicians have no respiratory protection and have not agreed to any exposure.
Hierarchy of controls
- Eliminate - do not spray this job in that bay. Wait for the booth or send the vehicle out.
- Substitute - there is no 2K clear of equivalent durability on the shelf that avoids isocyanate.
- Engineering - a booth with filtered supply air, slight negative pressure and extraction at the gun. Nothing in the general bay provides this.
- Administrative - booking, signage, safety data sheets, health surveillance and a trained exclusion zone.
- PPE - last line only: air-fed full-face respirator or supplied-air hood, coveralls, nitrile gloves, eye protection.
The 'ten minutes' argument fails because isocyanate exposure is not proportional to job size. A short, high-concentration cloud is exactly the exposure that sensitises people. Mix nothing until there is a compliant space.
2. The job is postponed until the booth is repaired. Describe the controls and the sequence you would follow from pre-cleaning the vehicle to clearing the booth, and explain what each step is controlling.
Read the worked answer
- Pre-clean and mask outside the booth. Solvent and masking adhesive fumes are not pulled through the booth filters, and dirt is removed before the vehicle enters.
- Check the booth first. Confirm extraction, filter condition, pressure balance and that the bake cycle works. A booth that looks clean but is out of balance pulls contaminated air across the wet film.
- Exclude everyone else and post signage, then fit-check the air-fed respirator - positive-pressure test, no leaks at the face seal - before any product is opened. This controls inhalation and eye exposure.
- Mix under extraction, keep lids on containers, and clean the gun in a closed washer. Open gun-wash trays are a major source of solvent vapour and skin contact.
- Spray, flash off, bake, then keep the respirator on until the booth is purged. Fresh clear continues to release isocyanate and solvent after it looks dry; the film is not safe to approach just because it is tack-free.
- Handle waste properly. Seal used solvent and contaminated filters and coveralls as hazardous waste.
- Log booth temperature and humidity, products and batch numbers, mix ratios and spray times on the job card.
Each step targets a specific route of exposure: inhalation through booth airflow and air-fed air, skin through coveralls and gloves, eyes through a full facepiece.