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  #1  
Old 03-15-2018, 11:38 PM
Aluminum Aluminum is offline
 
Join Date: Jul 2017
Location: San Jose, CA
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Default closed-loop spray booth

I set out to make a spray booth inside the garage, so I bought one of these MiCS5524 VOC sensor breakout boards and hooked it up to an oscilloscope to try out as air quality monitor. It turns out these cheap MEMS devices are ridiculously sensitive to a wide variety of organic solvents used in paints and primers.

Next I bought some carbon air filter media through eBay, rolled up some 1/2" chicken wire into a 4"x2' tube as a sort of intake manifold for the Harbor Freight blower #31810 (sans bag), wrapped the entire filter media roll around the chicken wire tube and plugged the other end of the tube with a piece of cardboard. I let this contraption run inside the closed garage, recirculating the air through the carbon filter.

Around the 1-minute mark on the 10-minute trace shown below I sprayed 2 ml of lacquer thinner into the ~2300 cu.ft. volume of the garage. The carbon filter removes the vapors quite rapidly. Eyeballing the amount of VOC residue measured after ten minutes, I estimate that this works about half as fast as venting to the outside through the man-door with the same blower, before any attempts to seal the leaks past the ghetto filter holder that surely reduce its effectiveness. The blower's stated CFM would recirculate one garage volume in about four minutes, so this all calculates, sort of.






There was only a faint trace of lacquer thinner smell left when I removed my mask after ten minutes. It doesn't seem like the VOC sensor or the carbon media miss anything that the nose can detect.

According to this document, activated charcoal can adsorb up to 25% of its own weight of certain solvents. The $40 mat I bought contains 600g of charcoal, so should be able to pick up about 150g of MEK, toluene, xylene, etc. before needing replaced. This is about half a gallon of high-solids paint, depending on paint, provided one catches the overspray droplets with a prefilter or paper towels.

Aquatic charcoal media are quite a bit cheaper at $1/lb but work much slower as the granules are large, so would require a better filter box design and/or grinding up into powder--not sure what the best way would be to get rid of fine charcoal dust, HEPA filter perhaps, or paint the airplane black lol.

The "Lower Explosive Limits" (LEL) of the most common solvents seem to be in the 1%-2% range, around 12-24 g/m^3. One would need to spray e.g. six pounds of pure toluene into the volume of my garage before creating an explosion hazard. This is a lot of paint, much more than one airplane's worth. The closed system appears quite safe at this rate of removal, unless my math is off.

Let me know your thoughts. While probably too expensive for professional work, to me this approach seems like a promising direction for our one-off airplane painting tasks. It would allow spraying in climate-controlled spaces, with much better control of temperature and humidity, and easier dust control. This combined with the neighbors' happiness will likely justify the extra cost for me.
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  #2  
Old 03-15-2018, 11:50 PM
lr172 lr172 is offline
 
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carbon does well with most aromatic solvents (i.e VOC's/volatile organic components), like laquer thinner and will help with reducing smell for your neighbors. However, they are not the risk when painting. They may give you a headache, but don't do real damage in short term exposure. It is the isocyanates in two part urethane paint that is the real monster and it is not filtered out with traditional carbon filters (it is not a VOC). You need the high end elements to filter this out and none of the providers specifically state protection from Isocyanates, so it's unclear how well these actually work, though they seem to be effective. Iso's have created some serious problems for some folks.

Just want to be sure everyone knows the risks associated with 2 part urethanes and are taking appropriate precautions.

Larry
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Last edited by lr172 : 03-16-2018 at 12:00 AM.
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  #3  
Old 03-15-2018, 11:55 PM
Aluminum Aluminum is offline
 
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Quote:
Originally Posted by lr172 View Post
It is the isocyanates in two part urethane paint that is the real monster and it is not filtered out with traditional carbon filters.
Noted. That's why I plan to use the PSX 700 system which contains no isocyanates and sprays beautifully without thinning. I can't smell any of its other components through the charcoal filter on my face mask.
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  #4  
Old 03-16-2018, 12:02 AM
lr172 lr172 is offline
 
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Is that a 2 part system? How are you finding the durability of the finished coat. Does it resist scratching and chipping well?

Larry
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  #5  
Old 03-16-2018, 12:17 AM
Aluminum Aluminum is offline
 
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Quote:
Originally Posted by lr172 View Post
Is that a 2 part system? How are you finding the durability of the finished coat. Does it resist scratching and chipping well?
It's a 2-part system, yes.

Seems as tough as epoxy, probably because it does contain some fraction of phenolic resin in the mix if I'm reading the documents correctly. One of the recommended uses is for floors, and I can believe it from my limited testing. The siloxane gives it much better resistance to UV and aggressive chemicals than polyurethane, and much much better than unprotected epoxy which decays rapidly in sunlight.

The only negative I found so far with polysiloxane paints is that nothing will stick to them except more of same. On the other hand, one can recoat with same paint many years later without sanding.

Note that pigments do look a different shade than inside acrylic clears, different index of refraction I guess.

Also, it cuts&polishes well, but not to the same high gloss as pristine paint, at least with the kind of polish for automotive clears I had at hand.
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  #6  
Old 03-16-2018, 06:31 AM
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bret bret is offline
 
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You will need to move large amounts of air to keep the overspray down, I built a booth with three box fans, and two large inlet filters in the walk in door, it was not enough. I originally had exit filters but scraped that idea and just moved the cars out of the driveway highly recommend a fresh air hood or full face 3M mask with organic filters and peel away sheets. Full face has a protection factor of 50, a half mask is only 10. I did Base CC and lived....but barley......wife wanted to beat me every paint session, and I was pulling air from the house and exit the garage door........
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  #7  
Old 03-16-2018, 07:22 AM
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DanH DanH is offline
 
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Quote:
Originally Posted by Aluminum View Post
Noted. That's why I plan to use the PSX 700 system which contains no isocyanates and sprays beautifully without thinning. I can't smell any of its other components through the charcoal filter on my face mask.
Interesting stuff. I wasn't aware of it. Thanks!

BTW, seems like the SG finish would be an obvious winner for a warbird paint scheme. Shoot all the base colors in PSX700SG and do the lettering and detailing in cut vinyl.
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  #8  
Old 03-16-2018, 09:30 AM
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airguy airguy is offline
 
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Quote:
Originally Posted by Aluminum View Post
Noted. That's why I plan to use the PSX 700 system which contains no isocyanates and sprays beautifully without thinning. I can't smell any of its other components through the charcoal filter on my face mask.
This is the same stuff we spray on our compressor packages at work for the oilfield, very tough and wears well under abuse.
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  #9  
Old 03-16-2018, 11:30 AM
Aluminum Aluminum is offline
 
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Quote:
Originally Posted by DanH View Post
BTW, seems like the SG finish would be an obvious winner for a warbird paint scheme. Shoot all the base colors in PSX700SG and do the lettering and detailing in cut vinyl.
This calls for more testing, to see if the vinyl would stick. Cured polysiloxane is so slick that masking tape curls up unless I wet-sand the paint. Perhaps one could wet-sand under decals and polish the exposed paint after vinyl adhesive sets. I should have some 3M vinyl lying around, will try some variations when I get a chance.

The other problem would be faster weathering than the paint. The vinyl I applied 6 years ago looks like cr*p compared to a good PU paint job from 24 years ago on a neighbor's RV: no gloss and wrinkled up around moving fasteners. I'm getting ready to remove it and repaint with the PSX paint.
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  #10  
Old 03-16-2018, 12:33 PM
SJordan SJordan is offline
 
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I?m in the middle of painting my RV-4 in my homemade paint booth and boy was I suprised. I have A 7000cfm fan pumping air through 16sq/ft of inlet filter and I can still fully fog up the booth with the SPI high solid clear. I cant imagine trying to close loop this air. If you look in th second picture you can see how dirty the inlet filters already are after just shooting base coat on the fiberglass pieces. Love the scientific approach to this though, don?t see this level of approach on the good old car forums.

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