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This thread shares information about my newly constructed knock down spray booth.

I should say up front that I am really not a sprayer. Not yet, anyway. I have been very hesitant to get into spray finishing, due to concerns about safety. I don't want to breath in finish, and I don't want to blow up my house. However, after French polishing 7 guitars (my first 7), and experimenting with a brushed finish on #8, I have gotten to the point where I really want to get a better finish with less work. So, I took a fresh look at spraying. The big hurdle for me was figuring out where in my shop to spray so that it would be both safe and effective. That took work. I did a lot of research and thinking this through, and I feel I have come up with a decent solution for my shop.

Some disclaimers: The solution I came up with should NOT be used in any space with a gas furnace. My spray booth's way of getting make-up air can cause serious health/safety issues in a space with gas heat. Luckily, I have electric heat. Moreover, my spray booth does NOT fully comply with NFPA 33 or the relevant OSHA regulations. While my spray booth has superior ventilation, it does not have other safety features required by those regulations, particularly regarding fire prevention and containment. While I use an explosion proof fan, I do not go full tilt on fire prevention/containment measures, instead relying on great ventilation to minimize risk. So, be careful before you adopt any of my ideas. Finally, ALWAYS wear the correct PPE when spraying. I know I do.

After looking at a lot of information about spray booths, I came to this conclusion: The critical function of any spray booth, the thing that makes the difference between success and failure, is the ability to move lots of air at the right speed. You need to move a whole lot of air through the booth, blowing the overspray-laden air out of the booth to the open air outside, and sucking new air into the booth from somewhere that has clean air to give. That means that my spray booth needed to be located close to two holes to the outside world that are not on top of each other. After stewing on this, I decided that I needed to locate the spray booth in my main workbench area, which is close to both a window (where I can spew out the overspray-laden air) and a door to my garage (where I can open both that door and the large garage door to provide the make-up air). Of course, this meant that I needed to go with an open faced benchtop booth, and I needed to make it so that it could be set up and broken down easily and relatively quickly, and stored somewhat compactly. And, again, it needed to be safe and effective otherwise, why bother?

This is what I came up with:







The booth itself has the following interior dimensions: 40" deep; 40" wide; and 30" tall. The box is made up of 5 lightly framed walls that I put together with my Kreg jig. I faced all of the walls with a 1/16" thick white plastic sold at Home Depot as an alternative to FRP (fiberglass reinforced panel), the stuff you see on the walls of gas station bathrooms. The walls are connected to each other with a combination of barrel nuts, T nuts, threaded inserts, and various knock-down furniture bolts. I can put the box together or tear it down in a few minutes.

Inside the booth, I have a few neat features. I stole John Hall's idea of using a rotisserie motor to slowly rotate the guitar (thanks, John) and "kicked it up a notch" by placing the rotisserie tower on top of a 24" rotating disc, facilitated by a 12" lazy Susan:



The combination of the rotisserie and the lazy Susan means I really don't have to move the spray gun around too much at all. I can maneuver the guitar body/neck however I need to.

The other neat interior feature is the removable LED tape light:



I cut the tape light to the right lengths and inserted corner connectors. I inlayed rare earth magnets in 1" wide strips of lauan plywood, and glued the strip lights to the plywood. The magnets stick to some of the washer-head screws that hold on the plastic wall faces. I can install or remove the lights in seconds.

The heart of the booth is the rolling cart that holds my fan:



The fan is a Dayton 12" tube axial fan, powered by a Leeson 1/3hp TEFC, single phase motor. My research convinced me that many DIY booths do not move enough air, so I put the bulk of my budget into the fan. Because my workbench is more than a foot lower than my window, and I wanted the filter box to sit at the bottom of the booth, I needed to tilt the fan up 15 degrees to get the exhaust to flow in a straight line (a fairly big deal for air movement). This took work to plan it out, but it is extremely effective.

On the booth side of the fan, there is a filter box that connects to the back wall of the booth:



It has interior dimensions of 20"x20" at the face, which is the most popular size for pre-cut spray booth filters. I'm using Paint Pockets for now. Many cheap HVAC filters come in that size, as well. Chicken wire keeps the filter at the face of the filter box, which helps to create a smaller version of what would be the exhaust chamber of a larger spray booth. That spreads the suction of the fan across the whole face of the 20"x20" filter.

On the window side of the fan, there is a chimney that I constructed from 5mm thick lauan plywood, with strips of pine at the inside corners to increase gluing surface:



The chimney goes up a 15 degree angle to the window, where there is a mating sheet of plywood to plug up the open window around the opening of the chimney.

So, how well does the spray booth work? Pretty darn well! Using a vaneometer, I clock the air speed inside the booth at an average of 150 fpm, ranging from a little above 100fpm to around 200 fpm. Must code compliant booths are supposed to shoot for a minimum of 100 fpm. For an open faced booth, a person really should try to exceed that number. In a fully enclosed spray booth, an undersized fan will eventually exchange the air; it just takes longer than it should. But in an open faced spray booth, the overspray can creep out of the booth and into the rest of the shop if the exhaust fan is not up to the task. So, bigger is better when it comes to fans for open faced spray booths. But it is critical to also plan for the provision of make-up air. You can starve the fan in a heartbeat by not providing enough make-up air. This is why having a door to an open garage nearby is essential to the operation of my spray booth.

This spray booth checks all of my boxes. It moves enough air to be safe and effective. It can be set up or torn down in a relatively short period of time. It has plenty of light. It has a great way to hold the instrument parts while spraying. And it can be stored in a relatively small footprint of space when not in use.

I can finally start the business of learning how to spray, now that I can do so safely in my shop. In case you are curious, I will spray waterborne finishes and some of the less harmful solvent finishes (shellac, Royal Lac), but I cannot ever see myself spraying nitro in my home.

I'm happy to answer any questions. Please note that this is still in the experimental stage, so some tweaks will probably be needed. If you have suggestions for improvements, I would be grateful for the input. Thanks.

Very cool, Don. I love the lazy susan. It will make the head and tail blocks really easy to get to.

What kind of gun did you go with?


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James—

I have Jeff Jewitt’s Qualspray QS-125WB, and I bought a stainless cup for it. My compressor is a California Air Tools 4620A, so not enough air for a lot of guns, but plenty for the QS-125WB. My tests with it so far show very little overspray, which is part of my overall plan. But again, I’m an infant at this, so we’ll see how things go.

I have learned a lot from Jeff Jewitt’s books and videos. He’s a great resource.

Paul Elkins -
He's really into coroplast - shelters, campers, boats, etc.
Here is his "rapid deployment shelter"
https://www.youtube.com/watch?v=qw18ELwQjgg
It looks like it might be adaptable to a KD spray booth.His other creations are very inspiring and worth a watch.
Another thing that might inspire an easily folded booth - a coroplast pop top (sans van):
https://www.youtube.com/watch?v=J7w4O0e1W-Q

Naturally I had to look that up, and the combo looks really attractive. I have an Iwata LPH-300 and sold my loud big box store compressor a few months ago to (ironically) pick up something from California Air Tools. Having a gun that would run well off a smaller compressor would sure be nice. Your setup has me thinking...


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Clay—

Yes, the material Paul Elkins used is certainly something that could work for the box part of a spray booth. This might be a good place for me to mention some earlier experiments of mine that did not work well.

I tried using two different sizes of the spray tents from Homeright. They are great for being a box for containing the overspray, and they have flaps in the back for matching up with a filter box. However, the flimsiness of the tent material made it hard for me to get a good, tight connection between the tent and the filter box without ripping the tent.

This was also the part of my experiments where I tried using a less robust fan, paired with some large diameter flex hose, for the exhaust. That didn’t work well at all. Many of you know how disruptive of air flow flex hose is; most folks who design dust collection systems try to minimize the amount of flex hose in a system, because it really kills air flow. That wound up being very true for me, unfortunately.

So, I decided to go with a sturdier box that can be reduced to flat, lightweight panels for easy storage, paired with a much stronger fan (what I use is comparable to the fans used in many commercial booths) and a much more direct and free flowing exhaust path.

having just made a booth myself I only have one thing to add.
Place some burlap along the sides and back. The overspray that hits the surfaces will build up rather quickly
burlap with all the surface area captures that.
Love the design lookes like it will serve you well.
Great job

_________________
John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

Thanks for that tip, John. I thought about using a filter wall for the whole back wall, for the very reason you cite. I decided to go with what I have and just clean the walls fairly frequently. Since this is a knock down booth, and the walls are sheathed in a type of plastic that is meant to be scrubbed clean, I am going to see if I can keep the overspray buildup under control that way. If it doesn’t work out, I will definitely move to having something absorbent and disposable on the walls. I appreciate the feedback.

Hi Don,
The coroplast would most likely be less rigid than what you have, but the quick set up and take down of those designs seemed attractive. The origami aspects of the pop top also I found intriguing.
A friend of mine uses a box fan with a filter box in front of it with the fan placed in an open window.
As John mentioned you will get overspray building up on the walls of your enclosure.Another option to limit that is paper. On the spray boards where I used to work they used a fire retardant paper to cover the board, and would periodically replace it when the overspray built up too much. "regular" paper would also work, and in either case you don't want to have too much build up on it before you change it out. If you only spray waterborne being flame retardant is probably not a concern.
http://www.blastlineind.com/Binks-29-86 ... 29-863.htm

Don, I know you are not trying to make the booth explosion proof because you will only be using water based finishes, but I would still recommend moving some of those electrical connections outside of the booth. I am mainly talking about the rotisserie controls and any other switches or connections.

Thanks, Barry. Yes, I want to do something about the electrical cords. I take comfort in the fact that vapor concentration in this booth is going to be very low (due to good ventilation), so the fire risk is likewise low. But, I want to minimize risk. I might put the cords inside a waterproof sleeve and control on/off via a switch outside the booth. I'll do some thinking about that. Thanks.

Don Parker wrote:
"So, I decided to go with a sturdier box that can be reduced to flat, lightweight panels for easy storage, paired with a much stronger fan (what I use is comparable to the fans used in many commercial booths) and a much more direct and free flowing exhaust path."

Since the spray booth is located in your main work area and you are pulling a lot of CFM with your fan you might want to consider adding a filter wall that could be put in place in front of the box after spraying the finish, but while it is still "wet" and setting up. If your shop is dust free it might not be necessary.
In my shop the guitar might wind up looking like a fur ball.

That's definitely on my radar as something to watch out for, Clay. When I decided to go with an open faced booth, I knew I would have to be mindful of dust getting in the wet finish. I'm going to try cleaning the shop well, and set up a used air cleaner I bought off someone not long ago, and see where that gets me. But I've gotten in the panel making business, so it is not hard for me to make another. Last weekend, I made a screen door for the doorway between my work area and the adjacent garage, so that I can leave that door open and open the garage door (for the provision of make-up air), but not worry about bugs getting in and our dog getting out. It works great! Thanks for the suggestion on the filter panel.

I have a small spray booth in my shop that has a 12" ducted exhaust. I originally planned to install air intake filters as well as the exhaust filters. Before I got the intake filters installed I had a guitar to spray so I just cleaned the shop first and sprayed it. Guitar came out fine. I do have a drying cabinet for the freshly sprayed guitars. Doesn't seem to matter, I just don't do any other work in the shop till the finish is dry. If t works for me and since I only finish a few guitars a year I decided not to bother with the air intake filters.

_________________
Steve Smith
"Music is what feelings sound like"

Thanks Steve; that's helpful to know. Yeah, I keep this particular part of my shop relatively clean of large dust and chips. Like any shop, there can be some airborne dust. My spray booth has a decent air flow (over 100 fpm inside the box), but it is not powerful enough to pick up heavy dust off any surfaces outside the box. If I can do a decent job of cleaning the shop, let the airborne dust settle, then clean again, and keep the air cleaner going before and after I spray, I am hopeful that these measures will minimize dust problems. Fingers crossed.

I thought I would share with the group some of the math relevant to the spray booth I put together. Lord knows I'm no expert, but I did a lot of reading so I could understand how to go about building my booth. The information might be useful to someone putting together something similar. Forgive me if the math and/or terminology is somewhat butchered. I'm not an engineer, and I don't spray for a living. My apologies ahead of time to all of those who really know this stuff.

In a typical negative pressure, cross draft spray booth (i.e., the air moves horizontally through the booth and exits through a filter wall, with a fan sucking air out of the room rather than blowing it in), various regulations require that the air move no slower than 100 feet per minute (fpm). Sometimes a faster air speed is required.

Since the air is moving toward a filter wall in a roughly linear fashion, one can compute the cubic feet per minute (cfm) required from the fan in order to move the air toward the filter wall at the desired speed, measured in fpm, in this manner: Take the area of the filter wall (width X height), multiply it by the desired fpm, and that is the minimum cfm you need from the fan. In my instance, the filter wall (not just the filter itself, but the whole wall in which the filter sits) is 2.5 feet tall and 3.33 feet wide. The area of my filter wall is 8.325 square feet (2.5 X 3.33 = 8.325). If I want to move air at a minimum speed of 100 fpm toward that filter wall, then I need a minimum cfm from my fan of 8.325 X 100 = 832.5 cfm. If I want to move the air at 150 fpm, I need 8.325 X 150 = 1,249 cfm from the fan. If I want to move the air at 200 fpm, I need 8.325 X 200 = 1,665 cfm from the fan.

Filters and ductwork add static pressure to any air movement system, and static pressure is a drag on cfm. Grainger, the retailer of the fan I wanted to use, provides some information about realistic cfm ratings, at various levels of static pressure. For the fan that I wanted to use (Dayton model 4C659, a 12" tube axial fan), Grainger's catalog says that, when paired with a 1/3 hp motor, my fan will move 1,767 cfm of air at zero static pressure, 1,667 cfm of air at 0.125" of static pressure, and 1,561 cfm at 0.5" of static pressure.

Once I put my spray booth together and ran some tests with a vaneometer, I saw that I was moving air at 150 fpm on average. 150 X 8.325 = 1,249 cfm. This tells me that my filter, the filter box, the fact that the air moves uphill at 15 degrees, any resistance from my chimney, and any resistance from the make-up air path, all together were adding enough static pressure to drop the fan's cfm from 1,767 cfm down to 1,249 cfm. But that's good, not bad. 1,249 cfm is right where I want it, because it gives me the 150 fpm I want inside the box. If I had gone with a smaller motor, the air speed might have been too slow, and a larger motor might have moved the air too fast.

I hope this is useful. Again, my apologies to those who really know what they are talking about.

Looks like I've got almost the same exhaust fan you have. Mines older but also an explosion-proof Dayton 12" tube axial. I am exhausting through 8 feet of 12" aluminum duct to the roof so I get a bit of loss there but overall it seems to do a good job. Hope your booth works out well for you.

_________________
Steve Smith
"Music is what feelings sound like"

Thanks, Steve. I’m interested in hearing more about what material you spray in your booth, and any other details about your safety or QC precautions. I’m glad I spent the money on my Dayton fan; it is what makes this design work. I bought it second hand, but unused, so it was a great deal, but still not cheap.

Hi Don, I put this booth together in 2012 and have sprayed EM6000, nitro, shellac and few others that I was experimenting with. The booth is only about 5' x 4'. I'm using a Walcom EGO HVLP gun with a 1.2mm tip and the 3M PPS system; I've had good results. In the booth and right before the hose to the gun I have an oil separator, a regulator and a commercial air dryer. I use one of those StewMac guitar holders mounted on a rotating platform.

The 12" tube-axial exhaust is ventilated to the roof and draws from a cabinet with standard HVAC filters to catch the overspray. The filter area is just under 5 sq ft. Electric inside the booth is minimized but what is in there is in waterproof conduit/junction boxes from the big box store.

The last time I rebuilt the shop (and hopefully for the last time!) I put all of the saws and big sanders with the dust collector at the end opposite the spray booth. A big air filter is hanging from the ceiling in the middle of the shop. The idea was for the shop to have a clean end and a dirty end. Even though the shop is only 20' x 15' that concept works surprisingly well. Good dust collection makes that possible.

A drying cabinet helps to keep the guitars dust free while the finish cures so I can go on with my other work. Admittedly most of my time is spent on repairs and setups so I'm not normally generating a huge amount of dust on a daily basis.

I've posted these before but maybe they'll help others get some ideas.

_________________
Steve Smith
"Music is what feelings sound like"

Don, one thing keeps bugging me about your booth. I think there will be restricted access when spraying a guitar inside the box. You can play it by ear and try it as is to see if this is an issue. But in the future, you might need to move the guitar outside of the box but directly in front of it to get full access. If that is the case, some angled side shields on the top and sides of the box would help collect the overspray. That is similar to my spray booth where I have the guitar hanging from the ceiling in front of my booth. It will still work fairly well.

Barry—

That is actually one thing I can say, with confidence, will probably not be an issue. I measured this very carefully. The dowel that is driven by the rotisserie motor fits inside the StewMac black metal spraying holders, which screw to the inside of the neck joint on both the body and the neck. This makes the body/neck stick out from the rotisserie supports far enough that, with the action of the lazy Susan, give me very good access to those neck joint access points and parts adjacent. The combination of the rotisserie and the lazy Susan work well at letting me get to everything that needs spraying. I measured the “wingspan” of both the body and the neck on the fixture, and built the box to give several inches of extra space beyond the full rotation wingspan of each part. It is good to be cautious about those measurements, though, and I definitely recommend lots of testing before building anything like this, to make sure it is big enough.

Can you control the speed on those fans? There may be times when you want to run them at a slower speed. If the filter box is removable they might also make a nice window fan.
I also use what looks like a cake decorating turntable similar to a lazy susan to revolve the guitar body on. It give a lot of access to spray the work while standing in one spot. Combining it with the rotisserie should make it that much better.

Clay--

I can think of only two ways of controlling the speed on the type of fan I am using: Use a different motor (a variable speed motor) or switch out the sheaves to gear down the rotation of the fan blades. That second option is not a great option, because the sheave inside the fan housing can only be so big; it is inside a housing, which is inside the fan tube. Not much room for growth. And the motor sheave can only be so small.

I have not looked into the possibility of using a variable speed motor. It would probably work, but it would probably cost some money.

I have thought about ways of using the fan, with the filter box (I can't separate those), but without the spraying box, for some other tasks. It would capture sanding dust pretty well, I think. I could use a different filter for that. Food for thought.

Or ditch the filter and blow the dust out the window - much more efficient! What's a little dust among neighbors?

If you want to throttle down the flow rate, it would be easier to have a butterfly valve or someway to obstruct the duct like a sliding gate valve, like they do on dust extraction ducting.