Sizing a gel coat spray booth correctly requires more than choosing a booth large enough for the part. Airflow, ventilation velocity, material usage, styrene/VOC emissions, filtration, make-up air, and fire protection all need to be considered.
For fiberglass and composite manufacturers, this becomes especially important because gel coats commonly contain volatile solvents such as styrene.
1. Start With the Part Size
The booth needs enough clearance around the largest product for spraying and airflow.
For example, if the largest part is:
14' wide × 12' high × 30' long
you might consider a booth approximately:
20' wide × 20' high × 40' long
The additional space provides room for painters, spray equipment and airflow around the product.
The exact clearances depend on the application and booth configuration.
2. Determine the Required Air Velocity
For many large conventional spray booths, 100 feet per minute (FPM) is a common starting design velocity.
However, the required velocity depends on booth configuration, spray equipment and crossdraft conditions. OSHA's spray-finishing ventilation requirements should be evaluated for the specific application.
For a crossdraft booth, airflow is generally calculated using the booth's cross-sectional area:
CFM = Width × Height × FPM
Example: 20' × 20' Gel Coat Booth
Cross-sectional area:
20 × 20 = 400 sq. ft.
At 100 FPM:
400 × 100 = 40,000 CFM
Therefore, the starting exhaust airflow would be approximately:
40,000 CFM
Notice that the 40-foot booth length isn't included in this calculation. For this type of calculation, we're sizing airflow from the cross-sectional area through which the air travels.
3. Don't Size a Gel Coat Booth on FPM Alone
This is where gel coat applications differ from a simple airflow calculation.
The ventilation system also has to control the vapors generated by the coating.
For example, an Interplastic White Marine Grade Gel Coat SDS identifies the material as a flammable liquid and lists styrene and methyl methacrylate among its ingredients.
The SDS specifically calls for adequate ventilation and engineering controls that keep airborne contaminants below applicable exposure limits and vapor concentrations below explosive limits.
Consequently, 40,000 CFM should not automatically be considered adequate simply because it produces 100 FPM.
The material consumption and solvent evaporation rate also need to be evaluated.
4. Determine Maximum Gel Coat Usage
Ask:
How many gallons per hour can actually be sprayed?
Suppose production requires:
75 gallons of gel coat per hour
That's substantially different from a booth spraying only 5 gallons per hour.
The ventilation analysis should consider factors including:
- Maximum gallons sprayed per hour
- VOC content
- Styrene content
- Other volatile components
- Transfer efficiency
- Number and size of spray guns
- Simultaneous gun operation
- Actual evaporation rate
- Applicable exposure limits
- Lower explosive limit (LEL)
For example if the gel coat SDS lists 30.8% VOC by weight.
At high production rates, this solvent load can become an important part of the ventilation calculation.
5. Check the LEL
Ventilation also has an important fire and explosion-control function.
The example gel coat has a reported lower explosive limit of 0.9% and upper explosive limit of 12.5%.
The ventilation system therefore needs to be evaluated to ensure flammable vapor concentrations remain safely below applicable limits throughout the booth and exhaust system.
This is one reason gel coat booth sizing shouldn't be based solely on air changes per hour (ACH).
6. Consider Styrene Exposure
Styrene can become one of the controlling factors in fiberglass manufacturing ventilation.
The example SDS lists occupational exposure limits for styrene, including an ACGIH value of:
20 ppm — 8-hour TWA
40 ppm — 15-minute STEL
The SDS also lists other OSHA and NIOSH values.
These limits are not interchangeable, and the applicable occupational-exposure requirements should be determined for the facility.
A booth could theoretically provide sufficient airflow for spray capture while still requiring additional engineering controls to adequately control employee exposure.
7. Size the Exhaust System
Once the required CFM has been established, the complete exhaust system must be designed around that airflow.
That includes:
Exhaust fan → filters → ductwork → discharge stack → make-up air
A fan advertised as "40,000 CFM" does not necessarily produce 40,000 CFM once filters, ductwork and other system resistance are added.
The fan needs to produce the required airflow at the system's calculated static pressure.
For example, a booth requiring 40,000 CFM might have resistance from:
- Intake filters
- Exhaust filters
- Dirty-filter loading
- Duct transitions
- Elbows
- Exhaust duct
- Dampers
- Discharge stack
The fan should therefore be selected from its performance curve at the calculated operating point.
8. Don't Forget Make-Up Air
If you're exhausting 40,000 CFM, approximately that amount of air must come back into the building.
Without adequate make-up air, the building can become negatively pressurized.
That can cause:
- Reduced booth airflow
- Doors that are difficult to open
- Uncontrolled outside-air infiltration
- Temperature problems
- Poor spray patterns
- Contamination
- Building pressure problems
For large industrial gel coat booths, a properly engineered make-up air unit (MAU) is often an integral part of the system.
9. Filtration Matters
Gel coat overspray can load exhaust filters quickly.
The filter system needs enough surface area to handle the required CFM without creating excessive pressure drop.
Filter selection should consider:
Airflow capacity + overspray loading + holding capacity + pressure drop + changeout frequency
A booth that performs well with clean filters still needs to maintain acceptable airflow as the filters load.
Differential-pressure monitoring can help operators identify when filter loading is affecting booth performance.
10. Example Gel Coat Booth
Consider:
Booth: 20' W × 20' H × 40' L
Maximum gel coat: 75 gallons/hour
Initial design velocity: 100 FPM
Cross-sectional area:
20 × 20 = 400 sq. ft.
Initial airflow:
400 × 100 = 40,000 CFM
So:
Initial design point = 40,000 CFM
But the engineering doesn't stop there.
The 75-GPH material load should then be evaluated against the gel coat's composition, VOC emissions, flammability characteristics and occupational exposure requirements.
The final required airflow is determined by whichever applicable design requirement controls.
The Bottom Line
For a large gel coat booth, a useful preliminary calculation is:
Booth Width × Booth Height × Required FPM = CFM
A 20' × 20' cross-section at 100 FPM gives:
40,000 CFM
But gel coat consumption matters.
A properly engineered system should also evaluate:
Material usage → solvent/VOC generation → styrene exposure → LEL → exhaust CFM → filtration → static pressure → fan selection → make-up air
That's why two identical 20' × 20' × 40' booths can ultimately require different ventilation systems.
Need Help Sizing a Gel Coat Booth?
Paintbooth.com designs industrial paint and gel coat booths around the actual manufacturing process, not simply the dimensions of the enclosure.
Whether you're spraying fiberglass boats, composite components, large molds or other industrial products, we can help determine the booth configuration, airflow, filtration and make-up air requirements for your application.
Request a custom booth design from Paintbooth.com.