Exhaust Fan for Paint Spray Booth

You can't make a regular blower motor explosion proof.

You have to buy one that is made that way.

As much as they cost. They are far cheaper then you losing your insurance if you don't use one.

Alternatives?

Then are there any alternatives to the "explosion proof" fans? Perhaps using one with a pulley drive and an external motor? How about a squarel cage type HVAC fan, where the motor is out of the airflow path?

I believe you still need explosion proof. As long as its in the room were if you don't turn the fan on right away. Fumes could build up.

Check your local building code. And or insurance company.

how about pressurizing the space and let the paint fumes go out thru louvered damper set up..you can put a potentiometer on the damper motor to control the pressure of the space with a supply fan running pumping the room up.the shafted fan to squirrel would definitly work and be safe that area the shaft connects the motor to the squirrel is always a negative during running so the motor is totally out of the paint electrical mix situation

The space probably wouldn't be air tight enough to only force the paint only out the opening.

This project sounds like you need to check the codes in force in your area. If you are building a booth, will it meet fire code and building codes? the paint booths I have seen had hte motor outside of the exhaust air stream, but I never asked if the motor needed to be rated for hazardous use. If you run explosive/flammable vapors through the motor, expect some excitment. your electrical switches need to be a certain distance from openings. Your lights need to meet certain codes, If the booth is heated have you made provision for fuel shut-offs? If you are in an IFC state, you need a fire suppression system. If you do not meet local/state requirements, your insurance company may not cover any incidents.