I've googled a bit about spray finishing ...
https://gfsboothblog.com/2013/03/26/...n-performance/
"It is more common for the ventilation performance of a spray booth to be specified by the average velocity through the booth or in the vicinity of the painter and product. From the design velocity you can calculate ventilation flow rates and air changes.
For example, if your booth is 16 feet wide by 14 feet tall and is a cross draft design, typical velocity in the booth would be 100 feet per minute (fpm). The total flow through the booth would be 22,400 cubic feet per minute (cfm) (16 x 14 x 100). The volume of the booth is 8960 cubic feet (16 x 14 x 40). The air changes can be calculated by dividing the flow rate by the volume. In this example, the air changes would be 2.5 air changes per minute (22400 / 8960).
These sorts of numbers equate to the air flow velocity in the one spray finishing booth I've used.
For a 8' x 8' x 20' container, your fan (after accounting for the resistance of all the particle filters and exhaust air cleaners would need to pull 1600 cubic feet per minute)
The link also gives an example for a down draft design and cautions that "the ventilation rate shall be able to maintain the concentration of flammable vapors in the exhaust below 25%."
and suggests that maintaining less than 25% may require additional air flow.
https://www.toolsusa.com/blog/how-mu...booth-require/"The real issue with air flow in a spray paint booth is having adequate air. A 14’ wide x 9’ tall x 26’ long spray paint booth will exhaust at least 13,104 cubic feet of air per minute. Running this spray booth for twenty minutes requires 262,080 cubic feet of available air. This requires a building at least 17,472 sq ft with 15’ ceilings or 10,483 sq ft with 20’ ceilings. This issue of an undersized building can be addressed with booth design and/or an air make-up unit. "
bearing in mind this is written for North America where, during winter, pulling sub-zero outside air into a building is undesirable, but the principle still applies
In terms of filters, How to Calculate Spray Booth Air Flow
Proper airflow is not only necessary for the protection of the operator, but is necessary to achieve the desired finish quality. Flow that is too low will not draw the overspray to the filters. Much of the overspray will end up on the part as dry spray (rough finish). If the flow is too high, solvent tend to evaporate too quickly resulting in dry spray.
To calculate the amount of exhaust air needed for the booth, a simple calculation is used.
CFM = Booth Face Area in square feet multiplied by the required flow (typically 100 FPM)
For example, and 8 X 10 filter bank (80 square feet) would require an exhaust of 8000 CFM (100 X 80) to achieve the required 100 FPM velocity.
and although not an NZ or Australian standard, the information here NFPA Ventilation Standards - Paint Booth Technologies - Industrial & Automotive Paint Booths should be a useful guide.
