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MG MGA - Air flow for painting car

Hi, I'm getting ready to paint my car in my garage. I've got all the inside covered with plastic, and I've installed a couple of furnace filtres for air circulation. My question is this: Given that my garage is 12 X 16 X 7 (1344 Cubic feet), how much air flow do I need in order to paint safely. It a 200 CFM exhaust system enough to safely paint the car or do I need more?
(I will of course also be wearing protective clothing and a mask)

Cheers, John
g gillian

Not even close. A standard small bathroom exhaust fan is 50 to 70 cfm. I have one in a room with a hot tub that does 300 cfm, approaching 1/2 room air exchange per minute. It's okay for keeping the humidity down, but nowhere near the volume needed for painting. For exhausting a paint booth you need to think in terms of cubic feet per second.

When I painted my MGA last year I put a furnace filter in one window and a 20-inch box fan in the other window for exhaust. I don't know the air velocity, but surely faster than I can run, maybe 20 mph at the fan exit. For a two square foot axial fan that would be 3520 cfm, or close to 60 cubic feet per second. It gave a reasonable cross draft for the room, but definitely NOT too much air flow. With continuous spraying the place would eventually fog up and drop paint dust on everything in the room. It would take a couple minutes to clear out after I stopped spraying.

My room volume was roughly 15x21x9 = 2835 cubic feet. Size of the room is almost irrelevant once it gets saturated with fog. You need enough exhaust air flow to carry away the over-spray dust and solvent fumes before it reaches high concentration. If you want to test your exhaust system, put a pint of lacquer thinner in the paint gun, stand to one side, point it toward the center of the room, and spray continuously until the gun is empty. If anything gets damp, or the room gets badly fogged up, it's not enough exhaust flow. For a large space I'd say at least one complete air exchange per minute, and maybe two air exchange per minute for a smaller room.
Barney Gaylord

Maybe this is obvious, but it's worth mentioning anyway. When spraying anything that contains flamable solvents, like pretty much any automotive paint, and using an exhaust fan to suck the contaiminated air out of the room, you should be using an explosion-proof fan. Electric motors in normal exhaust fans can spark, and a spark might be enough to ignite the air-borne solvent fumes.
Andy Bounsall

That also means don't operate light switches etc. while spraying and keep the compressor out of the room as well. You could use the fan on the supply air side where the air has no fumes.
Art Pearse

As Art states you blow through a filter to ciculate air. Two big advantages, 1 no risk of explosion, 2 As you presurise the room with clean air all the little cracks in the room only let out air. If you suck air out of the room all the cracks, like under the door, let in unfiltered air.
R J Brown

I use a centrifugal blower, like you find on a hot air furnace. Works quite well although I never really checked the flow rating. Also, I don't know what type of spray equipment you have but I highly recommend an HVLP spray gun. Several advantages. Uses much less paint, produces must less overspray and lays the paint down very well.
Good luck,
G T Foster

I have used a the blower package out of a standard forced air furnace for years. There not hard to come by and move a lot of air. As Barney says you will still fog up if you spray non stop, when it fogs up, step out until it clears. You will never be able to approach the kind of air volume that is achived in an actual spray booth, ours at work have (2) 50 horse power motors.
J Heisenfeldt

He. I guess we were lucky when we sprayed PopTop...


My box fan has an induction motor (slow accelerating), no brushes, no sparks. My 2-hp air compressor motor is an open frame motor with a capacitor start, also no brushes and no spark, but it did eventually get loaded up with dust and burned out, so I to replace it last year after 22 years of faithful service. My gas wall furnace has a sealed firebox, box no open flame, so I can use cleaning solvents when heating in cold weather. I don't smoke, I do weld (stick, MIG and gas), but no cleaning solvents used when welding. I haven't blown anything up with nearly 30 years in the same workshop, just use common sense.
Barney Gaylord

Barney, the compressor motor might be OK but the pressure on/off switch won't be.
Art Pearse

Pressure switch is in an enclosed housing. I wouldn't call it explosion proof, but it's better than being exposed.

With paint fumes in the air you need a fairly high concentration of "fuel" to make the mixture rich enough to be combustible. If you were to accidentally spray paint directly onto a hot light bulb it would burst the bulb from thermal shock. You would likely get a small local explosion within a couple feet of the ignition source. That might be enough to start a fire, depending on presence of other combustible materials in the area, but not going to blow the walls out of a garage with an open window or vent.

Otherwise moderate air flow through the work space should keep the fuel mixture low enough to prevent combustion even with an ignition source. I must say you'd be nuts to try painting with no ventilation.
Barney Gaylord

Guys go to your local paint supplier and ask who rents full heated spray booths, here in Aussie you pay about $30 per hour for a professional booth hire
Mark Mathiesen

This thread was discussed between 22/04/2009 and 30/04/2009

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