Building Your Own Spray Booth

Pros discuss ways to ventilate and filter. March 17, 2005

All of the recent posts on different finishes got me to thinking about my mediocre spray area. The room for finishing is a 12' x 24' room. I've built swing out drying racks on all the walls and in one corner I've got a 12" explosion proof fan with a box built around it to hold filter material. I then pull a plastic curtain across to keep the over spray out of the other area. This kind of works.

I cannot afford $2k-$5k for a professional booth and was thinking of building my own by doing the following: Build a false wall about 1' out from the end wall. Make this entire wall a filter wall. Then mount two 12" fans through the back wall.

A couple of questions - would two12" fans be enough air flow? Should I change over to one 24" or 36" fan? I've never seen in back of the filters in a spray booth so I don't know if that area is baffled to increase air flow or not. The make up air would come from the door into the rest of the shop. The shop and finishing room are on heat pump for heat and air. I would love to hear some suggestions.

Forum Responses
(Finishing Forum)
From contributor A:
I built an 8' x 8' panel outside of a garage door on the back of my building. It is mounted on barn door sliding track (it can be slid out of the way of the door opening). The panel is built out of 2 x 6 and 3/4" cdx plywood the 2 x 6 radiate out from the center of the panel for air flow. On the outside of the panel I attached a 30" spray both fan. The inside is open to the 2x6 and covered with chicken wire. I buy rolls of filter material and attach it at the top and bottom with a bat board. I put additional layers of filter material directly in front of the fan. I would like to add a speed control on the motor - it does not need to run full blast all the time, sometimes you just need to evacuate fumes when drying. Works great.

From contributor B:
I think regulations say exhaust has to go up a stack so it has to go up and out.

From contributor C:

You have a problem that is not uncommon to a lot of us. I live near Toronto. The local manufacturers advertise the fans and motors for sale all the time. I believe they are called tube axial fans and are usually belt driven. The fan is inside a tube with the motor mounted outside the tube. They come in a variety of sizes. If you check with your local body shops they should be able to point you towards the local spray booth manufacturers. An 18 inch tube axial fan will move about 3000 CFM and around here costs about $1500, Canadian. That price includes a 1/2 HP motor.

The booths that I have seen all blow out through a stack in the roof and are similar to your description. The wall of filters that remove the paint particles before they get to the fan are always at an angle.

From contributor D:
There are formulas for figuring air flow needed for booth fans. There are so many regulations for outtake of fumes and overspray that it would be wise to check Viking or some of the other spray booth websites for info. I'd give you the links but i'd just wind up banging my head on the computer again! Check the ads in the newspaper for used booths as this can be a less expensive route to go. Don't forget explosion proof lights in the booth also.

From the original questioner:
While I do believe in doing things safely (explosion proof fan and lights) I'm fortunate in that the shop is here on my land and don't really get into any inspector issues. I guess I'll look at some sites that might help me determine the amount of airflow I'll need.

From Paul Snyder, technical advisor Finishing Forum:
Since you're not dealing with any codes and are just looking to get decent air flow and catch the overspray, I think your plan has merit. Some things that come to mind looking at your plan:

* The wall should have an access panel so you can get behind it to get to the fan(s) or clean the area.

* You could use metal studs and cover them with a wire mesh to hold the filter media.

* Filter material in a roll will cover more area with less work to install or replace.

* Air in a booth usually moves at around 100 CFM per square foot. Multiply the width by height by 100 to get an idea of the fan size. For example, a room that's 12'w by 8'h would need a fan that can move 9600 CFM (12 x 8 x 100 = 9600)

* Placing the fan directly behind the filter wall may concentrate the air flow at the center of the wall. At least it seems like it would to me.

From contributor E:
I read CFM of fan should not be less the cubic foot in paint booth (room - in poor boyís case). My fan is 2500 CFM, my room is 250 sq ft x 10 ft ceiling = 2500 and it works great for me.

From contributor F:
As Paul Snyder stated in the equation for how much air you booth will move, the 100 CFM rating is a minimum requirement to insure safety. Also, typically there is about a 30" to 36" space behind the filters. This acts as a plenum which creates an even draw across the filters rather than just concentrating the flow around the fan opening. Consider also your input air. If you are removing 9600 cubic feet of air per minute, it has to come from somewhere. If you don't have enough input air you will strain the motors, like letting a piece of newspaper get sucked across the face of a window fan. It will also make it very difficult to open the door with the booth on.

Most booths that I see with an 8 x 12 foot filter area have a 30" to 36" fan. I think if you put a 36" fan in that room they will find you stuck to the filters. Think about reducing the filter area down in size so you can use a reasonable size (18") fan. One 18" fan will move slightly more air than 2-12".

From the original questioner:
I just looked up some fan data and found that in free air one 18" model moves around 4600 CFM. By reverse engineering: CFM/100 = filter area or 46 sq feet.

Does this mean my current 12" fan whose CFM is rated at 2000 in free air should work with 20 sq ft of filter area? Currently it is boxed in with the box covered in filter and no way does it get evacuated out as well as I'd like, although it is mounted above the spray table area. The room is 12 x 24 x 8 = 2300 CF.

From contributor D:
If youíre going to travel up this route with the booth why don't you consider raising the floor some and turn the area into a downdraft booth. There is nothing better than to have your overspray sucked down rather than across the room and over other pieces in room! Also some people like the overhead ceiling air input but that is more problem than it is really worth. Check out all your options first.

From contributor F:
If the fan is mounted overhead, in a small box, then it is acting like a bathroom fan - remember the plenum thing? Vapor and overspray are heavier than air so you are fighting physics from the get go. You are correct with your filter calculations. An 18" fan will draw about 100 CFM through a 6' wide x 8' tall filter area. Unless you are spraying big casework, that should work, no?

From the original questioner:
You guys are really helping. I've done a new drawing (Yes Paul, I use Sketchup - the best and easiest 3D cad I've ever used and not very expensive. Not a cabinet program but rather something that lets me design). Here's what I've done - stolen ideas from each of you.

Mount fan out the back of the building to save on interior space.
Build a 2nd "wall" 6 inches out with baffling to form a plenum.

Another idea that gave me pause - raise the floor say 6" and suck the darn stuff downwards. I'm I bit too fat to be standing on filter material - would you create say a 2' wide filter opening around the periphery of the floor?

From contributor G:
I would like to throw one thing at you just for consideration. Start dropping the word that you are looking for a used booth and you can more than likely find one cheaper than you can build one. Don't forget that you need to cut yourself a check for 45 dollars or more for every hour you are thinking, planning, and building it yourself. That can add up fast. If you get a booth that is already manufactured, you can be assured it has been designed with the right CFM fan and safety components and will have filter media made to fit it.

Also, insurance companies are getting very particular about things like uniform mechanical code compliance and UL listing. They hate insuring cabinet shops and will drop you in a second if they detect something that may cost them a bean. Once you are dropped, your rates will be much higher with the next company if you find one that will insure you. Whatever you do will certainly be better than what you have in your eyes, but make sure everyone else who has a stake in your business sees it the same way.

From contributor H:
Paul has the formula right for sizing the fan, except the per square foot is the cross section of the booth. Don't confuse that with the square footage of your booth. You want the air moving through the booth at at least 100 feet per min. 150 FPM would be better. The bigger the filter area the better. This is usually the end wall of a booth. So the air is hitting the filters at 100-150 FPM. Most booths have at least 2 - 3 ft behind the filter bank. Moving 9000 - 10,000 CFM needs plenty of room. When buying the fan, make sure it is sized for the losses in the ducting and filters. Don't figure free air size.
Use at least .25" of static .5" would be better. You can always slow the fan down but it can be really hard to increase it.

From contributor D:
The floor in a downdraft booth would be metal grating and there would be no filters there. The filtration would be a spin-off of a regular booths plenum with the filters running horizontal. The new drawing shows your fan below and behind filters whereas factory booths will have them venting up through a stack in the roof. I'm just wondering if the stack aids in suction rather than having the blades doing all the work themselves. I have a silencer on my fan motor which allows me to actually talk in a normal tone rather than yelling and going deaf. Booths are noisy buggers and this will take its toll on your hearing after awhile. Contributor G has some good points about insurance companies - even though youíre out in the sticks there is always someone looking to cash in on your dream.

From contributor I:
The first thing you want to be concerned with is booth pressure levels. You want to achieve a laminar flow in your booth, not a turbulent one. If you plan to use a makeup air fan blowing into the room, its CFM can not exceed the CFM of the air being sucked out or it will cause a turbulent condition, which will make your overspray cloud around you instead of being sucked up and out. As your filters become clogged, the amount of CFM being sucked out decreases while the input fan is still producing the same CFM, which will become turbulent. Therefore, you would need this fan to be on a variable speed switch so you could compensate for this as the filters become used. The object is to get your zero pressure as close to the intake wall as possible so the exhaust fan reaches for the air behind you to exhaust out, bring all the contaminates with it. It is a hard thing to achieve at all times and you won't realize it until after you start spraying and see the overspray hanging around you. The easiest way is to use a non-pressurized system, where you make the intake wall or door to accommodate filter media just like the exhaust wall. The intake opening should be the same square footage or larger as the exhaust opening. This not only eliminates the need for adjusting all the time it also filters the air coming into the booth from dust and contaminates. As the filters become more clogged you will hear the motor labor, like putting a piece of paper in front of a box fan. Be careful as to what media you use for filtering. Some dry filters can not be used with nitrocellulose paints and some waterborne products. Hope this helps in your design, it did with mine. I spent months researching this stuff, and I finally got most of it from a physics engineer.

One more thing you may want to consider is installing a dual stage filter on your exhaust wall. Columbus Industries makes dry filter media that is compatible with nitrocellulose lacquers, if youíre using them.

The first stage should be a 6-layer expanded paper/polyester, which will capture 98.4% of paint particles. The second stage should be a polyester filter that removes 99.4% of particulate. The front stage will need replacing way before the rear stage will. If you get a months use out of the front you will get 6 months out of the rear. Also be careful in disposing of your filters if you are using solvent based material. Some landfills and county ordinances require it to be labeled as hazardous waste, which will cost you big time to dispose of, while waterborne products are no problem.

From contributor J:
I made my own spray booth. I bought spray booth extension wings, a head and two legs 16'' deep screwed them to my block wall. I have a 20'' explosion-proof fan from Grainger in the center of the wall. I could not go up. Then I took aluminum angle iron glue and pop-riveted together to make a tee, and made a grid with 20'' x 20'' openings but in front of the fan approximately 36Ēx 36''. I put plexiglass on - this helps the whole wall draw more evenly. Positive pressure is the key.

From contributor E:
If you donít have to worry about codes, inspections or regulations, why filter the fumes at all? Why not just send them straight, directly, to the ozone?

From contributor I:

Codes, regulations and inspections are in place to protect not only you, but the insurance company, your neighborís health and the environment. Obviously the original questioner is genuinely concerned about these matters or he wouldn't have posted the question to us in this forum. He obviously lives in a place where such inspections and regulations are not enforced. I too live in a place where I am not regulated or inspected by fire marshalls. I live in the desert 50 miles out of town limits and my shop is on my ranch. My closest neighbor is 1/4 mile away, but I too took precautions to install a properly functioning spray booth all up to code, for my safety and the safety of the environment. I give kudos to the original questioner for at least trying to make a difference. I've been to a lot of paint shops that paint cars in town and I see guys cleaning their guns with lacquer thinner and then dumping it on the ground - talk about violations! And these are the guys that are supposedly legitimate with all the permits and OSHA approved spray booths.

From the original questioner:
Someone mentioned not being worried about codes. While I might not get inspected, I do believe in safety and not dumping stuff out into the air around my property. That may seem a bit old fashioned, but heck that's me.

From contributor G:
I, for one, am glad to see a topic like this and it is also nice to see a long one that isn't a flame war. And it is also good to see people helping someone who is trying to do the right thing. Have you seen how they make the fans explosion-proof? It is actually quite simple. They mount a motor outside of the exhaust tube and run the fan with a pulley and v-belt through a kerf notched into the exhaust tube to accommodate them.

From contributor A:
The paint filters - paper or fiberglass - only collect the solids, not the fumes, true? Aren't the fumes the part that is hazardous to the environment? I would like to be able to collect the fumes, as I used to live way out in the country but neighbors are moving in. Any options?

From contributor K:
I need you to clarify positive pressure versus zero pressure. To contributor I: You say that it is best to have it as balanced as possible. I have been hearing that positive pressure (I understood this to be an abundance of incoming pressure) is better, and yes, filtered is even better. It seems to me that if you have a sealed room and you introduce pressure, this would be a benefit to the exhaust fan. It would not have to work as hard to get the air out. Almost like pushing the air out instead of pulling it out. At any rate, you don't want to starve the exhaust.

I built my own portable open face booth and it works for me. It is almost 8' tall and 6' wide. I have is sized at 125 FPM. I can say that you do want as much filter area as possible. Mine seems to draw pretty evenly across the face. The larger the media area, the more efficient your booth and you can go longer between changes.

One thing that I would suggest that you start considering is lighting. Put in as much as you can. It will make a tremendous difference.

To contributor E: You need the filters to keep the fan clean.

To contributor D: Have you sprayed furniture or cabinets in a true downdraft? It seems like everything would have to be on wheels or carried in. How wide are the slats and the slots between the slats? How do you clean the pit?

From contributor E:
I was not saying *screw up the planet* by any means. I do think lacquer thinner is a good weed killer if you need to get rid of a little, and have some weeds on the drive that laugh at Round Up. I always thought that nitro lacquer dried so fast once exhausted, would it matter if it dried in a filter or in the air? Donít the vapors eventually rise to the ozone?

From contributor K:
To contributor A: Yes, the filters capture the particles. Code in my area states that the stack has to be 1.5 the height of the roof. So a 14' roof would need a 7' stack, or 21' above grade. I am told that this is to get the vapors into the atmosphere high enough that it disburses them. There are waterfall-type filtering systems that can help you with this, too. Got deep pockets?

From contributor B:
To contributor G: Well said. The question of collecting all the fumes can be done but it takes a lot of money for afterburners and the gas to keep them going. It is a refreshing attitude to see people who are responsible for themselves as well as others are concerned. My shop is fully regulated and inspected often. A lot of my frustration is the direct result of competition that operates without permits or regulations combined with the attitude of all the agencies to never want or have to enforce any laws. I don't understand why they just don't get the purchase orders from suppliers and check the users! I don't mind the people who do the occasional side job for extra money but I'm competing with full time businesses on this.

From contributor I:
To contributor A: You can get carbon filters that will absorb some of the fumes, but itís highly unlikely it will get them all. Contact your filter supplier and ask them what percentage they remove.

To contributor K: There is a need to control booth pressure. Some paint booth systems are designed to keep a slightly positive pressure inside the booth. This reduces the possibility of dirt or contaminants entering the booth from the outside atmosphere. You achieve this by using a make up air unit that forces fresh air into the booth. If you were using a 5000 CFM fan to exhaust your booth, you would need a 5500 CFM fan blowing in the booth that would give you a slightly positive pressure. There again as I stated previously you would need this intake fan on a variable speed switch to compensate for the filters becoming coated from contaminates and reducing the air flow to the exhaust which would move your zero point closer to you the sprayer, causing turbulence in the booth. Another way to overcome this is to use VFD drives (variable frequency drives), which detect the reduction of airflow through the filters and increase the RPMs on the exhaust and decrease the RPMs on the intake. This is quite an expensive task to deal with, which I donít think Art has in his budget. Other systems are required to maintain a slightly negative pressure, to prevent paint vapors from escaping from the booth and contaminating other work areas. A slightly positive pressure booth is also required if for example you have product or people coming in the booth as the operator is spraying. In a negative pressure booth, if someone opens the door it will cause a draft to come into the booth bringing in contaminates and dust, while a slightly positive booth would eliminate this. Itís basically a judgment call on the person building the booth as to what his preferences are and what his budget will allow. In my booth I am the only one coming in and out. I bring my product in that needs to be sprayed and close the doors. It is important in a negative pressure booth to make sure your doors a sealed very well or you will suck contaminates around the door seals, bypassing the filters.

To contributor E: If you are using a fan to exhaust your nitrocellulose lacquer, you most definitely need filters or your stack and fan will become caked with lacquer resin, which is an explosion just waiting to happen. Ever heard of spontaneous combustion?

To contributor D: I understand your frustration completely. Itís the nature of the beast I suppose.

From contributor J:
One other point is that if you are building a booth my understanding of the code is that the wall material must be fire proof. In other words all walls of the booth should be made out of metal studs and sheet metal or some other material that will not burn. In addition you must have at least 3' clear around all the outside of all walls of the booth. If you are building a new room then you may want to take this into consideration just in case some day you want or need to be legal.

I believe it is also recommended to coat the inside walls with a special paint that can be peeled off once the walls get built up with finish. If you set the codes aside for the moment why would the following not work just fine?

Since both box fans and furnace filters are pretty cheap my thoughts are to build a wall that would roll in front of an open garage door that would have one box fan per 20" x 20" furnace filter. Just build in as many box fan/filter combinations that you would need. The wall could be rolled out of the way when the fans are not being used. When in use just spray in front of the filters with the box fans turned on.

Certainly these fans are not explosion proof nor do they have a CFM rating that I am aware of but it does seem that it would move enough air that you should never get to the point where the fumes would be concentrated enough to cause an explosion in the first place.

I believe that there are also devices that are commonly installed in booth that will measure how much air flow is actually being moved. Perhaps someone can describe how this works.

I believe that there is a point where you are not required to use a booth at all but I am not sure where the line is drawn. Someone using a spray can of lacquer in his home garage with the door open may not actually be illegal as an example. I believe that the line is drawn based on how much spraying is done per session but I do not know where this line is drawn. Perhaps someone can advise?

Too bad there is not more advice for the smaller shop that does not have the space or funds for a fully legal booth and permits but would like to do more then just spray in the open with no fans at all! My understanding from my ML Campbell supplier is that there are a lot of shops in my area that have no booths at all.

It seems that the primary consensus is that you should have a fully legal UL listed commercial booth with permits, etc. or you should use water based product. Seems like the majority of the smaller shops out there operate outside of this consensus with little guidance and with their fingers crossed! I shamefully and reluctantly admit I am in this category as well. I would also venture to guess that many others who are following this post are also in this category as well but do not want to speak up for fear of reprisal.

From contributor M:
One thing I found when building my own booth was that the theories I acquired beforehand only took me so far. Until I got in it and sprayed I didn't really know how effective it was. To this end you may be well served to leave variables in your incoming ventilation which you can adjust and tweak before you commit to a final design. I sized the exhaust fan according to the 100 CFM/1 SF rule and found it excessive. I replaced the drive pulley and belt to slow it down to about 2/3 this and preferred the in booth experience even though it seems to contradict the recommendation. I have a large intake opposite the ventilation van/baffles about 2/3 of the length of the booth away. During operation there is a stream of high velocity air (cold and turbulent) which flow from the intake to the outtake. I keep work away from this stream but shoot in its general direction allowing overspray to be captured. I am able to control the rest of the booth turbulence by opening and closing the door from the shop to the booth and the door from the booth to the drying room. Using these two variables helps when spraying different sized pieces. Leaving the booth to run for Ĺ hour or so after I'm finished, with the drying room door cracked open an inch or so, helps evacuate the initial flash off of solvents in the drying room. Also, we framed a stud wall 3 feet in front of the fan intake and mounted cement reinforcement wire (6" x 6" grid), to support the paint arrestor pads - cheap and fast to change or shift around as they clog.

From contributor I:
To contributor J: Yes, they have a device called a velometer that measures air flow. I have seen lots of guys on the web displaying their crafty inventions of a spraybooth with plastic sheeting and multiple box fans and furnace filters. I don't know if I personally would go that route, but maybe it will work and maybe it won't over the long haul.

As far as the small shops and the amount they spray I have been on the EPA's website and have read material concerning businesses that shoot over 5 gallons an hour - they fall into a certain category. But I am not sure about the little guy who shoots 5-10 gallons a week.

From contributor K:
To contributor J: The booth that you describe is in essence what I have. I'll try to take a picture of it next time I have it set up. I have two walls set on piano hinges and a piece of ply on top. It makes a small but nice portable open face. I do, however, have a hazardous location fan venting the fumes. And yes, I do know the difference between explosion proof and hazardous location.

As far as regulations go, you have two groups to deal with, at least in my area. One is the city fire department, and the second it the environmental group. With the city, we are allowed to spray without all the UL/lawyer crap if it is less than 9 sq ft of area. The state environmental group has a limit on the amount of material that you spray. Under a certain amount you are considered conditionally exempt as a limited generator. I haven't gone back to ask them what that limit is. I don't want to give them an invitation to visit my shop. As for the meter, I have heard it called a manometer.

From contributor I:
To contributor K: Two questions for you. First, you say 9 sq ft. and under is not regulated in your city - 9 sq ft of what? Filter area or product area that you are spraying? Second, what class is your fan - I, II, III? And how big is it?

From contributor D:
Yes, I have worked in a downdraft booth and this set-up is the best by far. You don't have to deal with air turbulence flowing across all pieces that have been sprayed which eliminates overspray. The grating sizes were 1-1/2' W x 3' L each, times 15-20 grids total for width of booth and 5 grids for the length of booth. The gridworks that you stand on and set horses on were about 1/2" wide by about 3" in length within the dimensions above. These set into gridwork for (steel framework) strength and support for each grid panel. We used to scrape off buildup or just drop them on the concrete to jar coating off grids. An explosion proof vacuum was used to sweep up powder left at bottom of grid floor and, of course, there were filters near the exhaust plenum that needed to be changed. Youíre correct about the term manometer. This measures air flow into the booth and is used to indicate when filter media needs changing.

From contributor N:
I would just keep your eyes open for a used booth. I donít think your going to save any money building one when you factor time and stress. And it probably wonít work as well anyway.

From contributor D:
WOODWEB has a great product directory and machinery finder located in the site map. Or do a search for spraybooths online.

From contributor K:
To contributor I: It is the object being sprayed - 9 sq ft. As for my fan, I can't remember. It has non-sparking fans and a total enclosed motor, even the wiring. I bought it at the flea market where a guy was selling damaged Grainger stuff. I whacked the flange with a mallet and it was as good as new.

As far as the grates go, it sounds like these were units that lifted out for cleaning. I wonder if there is a poor man's (i.e. - cabinetmaker) way to make these. How deep was the pit, and how big was the plenum?

As for buying used, I have been told that you have to be careful because some of the cheaper units that are held together with sheet metal screws, and they can wobble out (sorry if I am getting too technical) a hole and not hold any more. I was also told that the panels can warp and twist and it is the dickens to get them back together.

From contributor I:
To contributor D: Are you sure the booth that you used to work in used a manometer in it? A manometer is primarily used for measuring pressure, mainly in the liquid form, i.e propane gas or the such, while a velometer measures airflow (velocity) which is installed in front of the filters of the exhaust side of the booth and some install them in front of the intake filters inside the room to measure how much airflow is coming in. Just wanted to make sure in case someone reads these posts and goes out to buy one and finds out he has the wrong gauge. I don't have either so I can't say for positive which is the correct one.

To contributor A: 9 sq ft is not very big at all, rules out a dining room table I guess. I did some research on hazardous vs. explosion proof after reading your post. The only thing I could find that related to hazardous location fan was something that looked like a swamp cooler that had water flowing over the filters. Grainger classifies their fans in 3 categories, class I Explosion proof, used in areas where vapors, fumes and gases are present, class II was used in areas that have grain dust, or dry cleaning fumes and such, and class III was used in textile mills where clothing dust is present, or wood shops where the saw dust is present. I would venture to say that your fan is a class I explosion proof fan. The main thing with fans is keeping the motor windings out of the path of the fumes and overspray, this is what would ignite a fire, or the blades hitting a metal object and sparking a flame. I've seen crafty guys make their own fans by mounting motors outside the duct of the exhaust and have a belt go thru the duct to turn an aluminum blade fan that is mounted on a pulley. This should work just fine and would be much cheaper in cost versus an axial fan or a class I fan.

From contributor D:
Iím absolutely positive on the term manometer. It is a gauge that sits on the outside of your booth with a hollow needle screwed through the stainless wall of the booth near filter bank. It's filled with red fluid and adjusts when filters are clean for air flow. The gauge will increase in increments as filters clog up.

From contributor I:
To contributor D: I understand now, you guys were measuring the pressure in your booth, not the velocity of the wind flowing across the filters. You must have had a sealed booth with a make-up fan? You can get a velometer that has a pendulum swinging arm with vanes on it to measure the flow across the filters - as they get clogged the they needle starts to decrease, and this is when you know you need to change your filters.

From contributor D:
Actually this measuring device is the only one that I have ever seen for this application, whether it's in closed or open faced booths. That doesn't mean yours is not a proper measuring tool - it ultimately comes down to whatever works and is safe!

From contributor L:
Nine SF is pretty good size. The side of most cabinets is generally less then 5 SF. Of course you may be doing 20 or more pieces at a time but it sounds like it is per piece, not the total SF you are spraying! Perhaps this is how some shops get by without a booth.

From contributor O:
There is a FWW book on Spray Finishing that has good information, details and a plan on spray booths. Check it out for what it is worth - Spray Finishing by Andy Charron.

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor S:
Collecting overspray in filters is to aid in the disposal of explosive dust. The dried overspray dust is more explosive than the wet lacquer or the spray-mist. The filters collect much of this by-product, and any other dust which collects in the spray area should be cleaned out regularly.

Comment from contributor P:
I am currently working on a downdraft model paint booth. It measures 3 ft long, 2 ft wide and 2 ft high. The bottom is a full 2x3 grate with a 1 1/2 inch drop before the fan which recesses an additional 6 inches. My grates will be covered with a thin-glass but effective filter which will not restrict the airflow of the fan. I am using a 230 cfm for the down. Furthermore in the center length back of the booth will be an additional fan to remove direct spray in addition, but at a moderate speed setting.

It is of utmost importance to restrict with filtering all expelled material from entering the atmosphere. I have worked in high tech booths all my life and know the importance of clean environments when it comes to painting. For a healthier atmosphere and lungs, take the time to incorporate a filtering system in your booths.