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Hi DL, That's a tall order. I don't want to discourage you but will provide some of my thoughts for your consideration: 1. The "standard" for paint spray booth ventilation is to have an average 100 fpm air velocity across the face of a cross draft booth or across the floor area of a down draft booth. To put this in perspective, think of a cross draft booth that is 8ft high x 10 ft wide x 24ft long. The cross sectional area is 80 sf and the volume is 1920cf. To obtain an average 100fpm air flow would require a fan volume of 8000 cfm. That's a lot of air. 2. If you plan on doing painting in such a booth during the winter, you will need to provide a provision for heating the incoming air. Assuming an outside temperature of 20degF and a flow of 8000 cfm, that equates to a heating rate of 432,000 btuh that would have to be supplied to the incoming air in order to maintain a booth temperature of 70 degF. That's a lot of heat. Keep in mind that a flow of 8000 cfm across that volume is the equivalent to 250 air changes per hour. That's an awful lot. 3. A "standard" paint spray booth will have all explosion proof lighting and fixtures, including an explosion proof fan if the fan is in the air stream. It will be constructed of fire resistant materials and have an installed fire suppression system, such as automatic sprinklers. Do you need all this? I don't know. A lot depends on 1) what your local building codes may require, 2) your insurance company, 3) the attitude of any "nosey neighbors", 4) whatever safety risk you are willing to bear and 5) whatever you wish to avoid telling prople that could cause a problem for you. It may also depend on whatever local environmental regulations may exist that could be applied to individual operations. 4. The primary safety risk, in my opinion, is explosion due to flammable materials (solvents) being atomized in the air and the possibility of a spark, no matter how remote, even from static electricity, of causing a problem. That's assuming your breathing system consists of a fresh-air-supplied respirator in the first instance. I believe the NFPA (National Fire Protection Association) has concluded that in order to be "safe" from explosion, the air concentration in the booth must be kept below about 25% of the lower explosive limit of the solvents being sprayed. They (NFPA) have also concluded that a ventilation rate as noted above (100 fpm across the section) will be sufficient to be assured of safety from an explosion standpoint - for all reasonable rates of solvent evaporation. However, that being said, such a determination is not considered sufficient to eliminate the need for the explosion proof character of all the electrical components. As with all fire/explosion protection standards, there never is a reliance on a single level of protection. There are just too many unforseen circumstances and the consequences of a failure are too great. 5. That said, I have painted a few cars in the past in my garage/workshop using plastic film on the ceiling and sides to control the overspray, two 20" cheap window fans in the garage door, and a furnace filter in the window opposite the door. No explosion proof anything and no fire protection except for your standard fire extinguisher. This is really doing it on the cheap and I really cannot recommend it to anyone. I stopped the practice altogether after seeing all the overspray splatted over the open windings in the window fans and in the open motor on my compressor. To me, doing it that way was too great a risk to just keep doing it. Others seem to do similar things and get away with it. I haven't heard of any DIY garage being blown up because of paint spraying. But I do think it is a risk, considering the "official" standards for an "approved" spray booth. What little painting I have done in recent years, I have done completely outside. You'll have to be the judge of how you wish to continue. 6. The NFPA standards for such facilties are not available on-line without payment ($$$) but there are also OSHA federal requirements for "approvable" paint spray booths that are available. I probably can link to them if you want to study the matter a little more. 7. You might also want to read the thread down below (my original post titled "Ominous Sign") concerning possible future EPA regulations regarding the sale of automotive paint products. It appears that consideration is being given (by the EPA) of restricting the sale of auto paint products to only "certified" individuals that use "proper" facilities. The product labels already say "For Professional Use only...Not For Sale To The General Public" and have for a long time. A Google search on "epa paint sales", without the quotes, will yield some discussion on that matter. Whether such hard restrictions will ever come about is unknown at this time but is something to think about. It certainly happened with R12 some years ago.  Rod
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