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closed-loop spray booth

Aluminum

Well Known Member
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.



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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.
 
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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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.
 
Is that a 2 part system? How are you finding the durability of the finished coat. Does it resist scratching and chipping well?

Larry
 
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.
 
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 :rolleyes: 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........:eek:
 
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.
 
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.
 
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.
 
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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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.

Wait, what? You say your INLET filters get overspray!? :eek: As in, fresh outside air coming into the booth?

What kind of spray gun?
 
Make sure your activated carbon media/bed is adequately sized. There is always an equilibrium between adsorption and desorption. There is a narrow band in a carbon bed where the actual adsorption takes place. Upstream of that, the bed is spent and partially desorbing the stuff that is less strongly adsorbed. Downstream of the band, the fresh carbon is "polishing" what has been desorbed. Eventually the band will progress to near the end of the bed and you will begin to get breakthrough and the bed will be soon exhausted.

Depending on the vapor loading, bed mass and depth, and the superficial velocity, the heat of adsorption, if not carried away fast enough by convection via air or conduction through the bed, can build up rapidly and spark a bed fire. That's a bad day.

Make sure you contact your activated carbon manufacturer for proper sizing data.

P.s. charcoal and activated carbon are two very different things, but the terms are often nused interchangeably. Charcoal actually has very low adsorptive activity and is sometimes a precursor material in the AC manufacturing process. Medical applications (such as the carbon slurries used in emergency poisoning treatment) often improperly use the term "activated charcoal" to refer to activated carbon.
 
Depending on the vapor loading, bed mass and depth, and the superficial velocity, the heat of adsorption, if not carried away fast enough by convection via air or conduction through the bed, can build up rapidly and spark a bed fire. That's a bad day.

Very interesting! A quick googling for "carbon bed fire" comes up with great bedtime horror stories. Clearly this calls for further study.

First question that comes to mind is what's the best way to dispose of the saturated carbon media?
 
Looks like your inlet air has too many places to go. Doesn't flow through your booth like a river. Just a suggestion.
 
Fans

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 :rolleyes: 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........:eek:

And with all of the thinking going into this I see nothing about what kind of electric motors are being used on the fans. They should be explosion proof motors or all of the other bs could be just burned wood and garage if not used. I painted aircraft years back in the late 60s and even the lights and switches were done that way. You will need more area going in than out and also the exit has to be at the opposite side from the inlet or you wont remove much overspray.
 
Very interesting! A quick googling for "carbon bed fire" comes up with great bedtime horror stories. Clearly this calls for further study.

First question that comes to mind is what's the best way to dispose of the saturated carbon media?

"In the industry" lots of activated carbon is reactivated. It is very similar to the final steps of the production process for "virgin" carbon. High temperature (1600 - 1900F) in a reducing atmosphere in the presence of water vapor. Not easy to do at home.

At home however the spent carbon can probably just be burnt as fuel. I wouldn't heat a house with it unless iI was monitoring the process very well (high temps are necessary to destroy the adsorbed materials). And I certainly wouldn't cook with it. But it could be put on a campfire probably. You might want to try a small amount first to see if it smelled unpleasant...

Pure solid carbon has a higher heating value of about 12000 Btu/lbm plus you would add the heating value of the adsorbates, so could be as good or better than coal, depending on the adsorbates.

All statements here are mine personally and not representative of my employer.
 
This calls for more testing, to see if the vinyl would stick.

I applied a small patch of 3M automotive vinyl to pristine polysiloxane topcoat several days ago. Rubbed only, no heat gun. Today I tried lifting corners with 90 psi shop air. Good news: boundary layer won the battle. If I lift a corner using fingernail it peels off easily--adhesion is low but may be enough. It appears that vinyl decals over siloxane paint could work. Will revisit after flying with a decal for a while.
 
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