Applying finish is part and parcel of pretty much any woodworking project. For most of us, it’s more like an afterthought; something we have to do to bring out projects to completion; but not something we enjoy doing. While there are plenty of us who still use a paintbrush to apply our varnish and other finishes, any woodworker who does a fair amount of work will eventually turn their thoughts to spraying. Being able to spray finishes saves time, as well as providing for a smoother finish.
But spraying finish has its own problems to deal with as well. Sprayed finish tends to go everywhere, not just onto your woodworking project. Even if you try to be careful, chances are pretty good that you’ll end up with finish sitting on your workbench, tools and any wood you have stocked in your workshop.
The solution to this problem is to build a spray booth. This is an enclosed area, with filtration and a high amount of air movement, which allows overspray to be captured and prevents its spread. At the same time, a well-designed, built and maintained spray booth will provide a dust-free environment, improving your surface finish, by keeping dust off the project when you are applying the finish coats.
Spray booths can range in size from something the size of a medium sized shipping carton up to the size of a garage. We had spray booths that were 50 feet long, 18 feet wide and 14 feet high in the bus manufacturing plant where I used to work. That’s probably a bit more than anything you or I would need for our woodworking projects.
It’s All About Air
The key to any spray booth is air handling. More than anything, the idea is to draw away overspray and then remove the overspray from the air being moved through the spray booth. This requires a large amount of air movement, as well as some really good filtration.
Generally speaking, you want 100 cubic feet of air movement, for every square foot of spray booth opening. So, if you’re building a small booth type spray booth, with a 4’ wide opening and 3’ of height between the table (bottom of the opening) and the ceiling (top of the opening), you’ve got 12 square feet of booth opening. That works out to needing a blower which will provide 1,200 CFM of air movement.
When designing the air handling system for the booth, you must be careful about size. A blower which is capable of moving that 1,200 CFM of air needs a duct that’s roughly 12”x 14” or 14.1” in diameter (for a round duct) to push that air through. If you use a smaller duct, it will either need to increase the air velocity or won’t be able to draw that much air through the booth. But there’s only so much faster that a blower can push the air, before it starts losing the ability to handle the volume.
Please note that the sizes mentioned are also the minimum sizes for the filters being used to filter the air coming out of the spray booth too. It doesn’t hurt to make them bigger, at least double that size, especially since the air filters will gradually become clogged by the paint overspray, limiting the amount of air that can pass through them, just as a dirty A/C filter will limit the amount of air coming through your home HVAC system.
Build a Plenum
The air being drawn out of the spray booth is collected in a box, referred to as a “plenum.” This provides someplace to mount the air filters, as well as someplace to connect the blower. You want the face side of the plenum to be as big as the air filters you are going to use, providing a track or clips to mount the filters themselves.
For filters, you could use normal home A/C filters. However, you’ll be better off using HEPA filters, as they provide a finer degree of filtration. At some point along the way, the air from your spray booth is going to have to be dumped outside. If you don’t have good enough filters, you’ll end up dumping overspray out there, which will discolor your home, driveway, plants or lawn.
Connect the Blower
Pretty much any blower will work for your paint booth; but the easiest and cheapest option is to find a used blower from the air handling unit (the part that goes inside the home) of a home HVAC system. Rarely do the blowers go out on these systems, so if you can find one that has been removed, you can get a blower cheap. Make sure you take note as to the size of the air conditioning system you’re getting it out of, or the air flow. One or the other (preferably both) should be listed on the equipment’s information label.
As a rough figure, each ton of air conditioning works out to 400 CFM of air. So if you find an air handling unit for a five ton air conditioner, you’re going to have a blower that can handle 2,000 CFM of air. That’s probably a lot more than you need, unless you’re making a really big paint booth.
In that case, you can size your ductwork for the amount of air that you need to have pass through the system, rather than the size the unit is rated for. Use the chart in the footnotes to determine what duct size you need. You might also want to consider adding a light dimmer switch to the blower, so that you can control how fast it is running, which affects how many CFM of air it is drawing.
You’re going to need to connect your blower to the plenum. Ideally, it should be physically mounted to the plenum, so that the air input is attached to the plenum. If you can’t attach it directly, be sure to use a large enough duct, so that it doesn’t hamper air movement.
On the other side, the output of the blower needs to be ducted to outside of your workshop. Again, you want to avoid cutting down the size of the duct down to the point where you aren’t getting enough air moving through it. If you have a size problem, split and use multiple duct lines, rather than just reducing the size.
Add Some Baffles
As I mentioned a moment ago, overspray will clog up your filters fairly quickly. To slow this, all you need to do is add some baffles in front of the filters, forcing the air being drawn out of the booth to make a number of turns as it snakes its way through the baffles. This will cause a large amount of the solids in that overspray to stick to the baffles, before getting to the filter.
Keep in mind that the total area of your holes must meet the area of the ductwork, or the baffles will create an air restriction. Lay out the pattern and calculate the total area, before beginning to drill, just to be sure.
To make baffles, use any thin material, such as luan plywood. Make a grid of holes in two to three matching sheets. Be sure that the holes don’t line up though. You should only be able to see solid wood through the holes in the first sheet and likewise looking through the holes in the second sheet.
Mount these baffles one in front of the other, roughly one inch apart, and at least one inch in front of the filters, in a movable or removable frame that sits in front of the air filters. Make sure that it is sealed all the way around, so that air movement must be through the baffles, rather than going around them.
Don’t worry about buildup on the baffles. You might need to clean it off occasionally; but that’s not really much of an issue, unless it gets to the point that the buildup is reducing airflow.
Incoming Air Filters
Better spray booths will not only control air going out, but air coming into the booth. This is done with automotive spray booths and serves the purpose of controlling the amount of dust that can get into the spray booth and thereby into the finish. While not a requirement, if you have a dusty shop, this is something you might want to consider.
I worked with a guy a number of years ago, who turned his home garage into a spray booth for auto body repair work. For his incoming air filters, he removed a couple of the panels from his wood garage door, leaving just the framing. The panels were replaced by frames, which held his filter medium for the incoming air to the garage.
When adding incoming air filters, make sure that they total the same amount of surface area as the outgoing ductwork. You don’t want more and you don’t want less. By having the same amount of surface area, you’ll be able to maintain a slight negative air pressure in the spray booth, helping to ensure that overspray goes through the baffles and outgoing filter. But if your incoming filters are larger than your outgoing ones, you won’t have this negative pressure and some portion of your overspray will settle in the workshop, rather than being captured by the filters.
Making the Booth Itself
As I already mentioned, spray booths can be of any size and can be built of almost anything. More than anything, it depends on how much space you have available and the size of the items you will be spraying. If all you’re going to do is spray the occasional metal bracket, you don’t need much; but if you’re going to be finishing whole pieces of furniture, you’re going to need something the size of a small room.
I’m going to take a quick look at three different spray booth ideas here, showing you what can be done. I expect that you’ll modify these ideas, making something that will work best for you, rather than trying to build them exactly as I mention here.
Small Spray Booth for Brackets
If all you’re ever going to need to spray are small metal brackets and turned wood pens, you can get by with making your spray booth out of a plastic storage bin. This is a convenient tabletop size, although it doesn’t give you enough room for larger projects.
This size spray booth isn’t going to need a bull-blown A/C blower to move air through it. You’re better off scavenging a blower out of a window a/c unit. Some people have made spray booths like this using a bathroom fan, but that’s not a good idea. With a bathroom fan, the fan motor is inside the air plenum, where a spark could ignite the VOCs coming off your spray. You’re better off with a blower that has the motor outside the air plenum, such as the aforementioned blower from a home HVAC system.
Another option for this size spray booth is the blower from a car’s heater and air conditioner. These are normally located under the dash, on the passenger side. You might have a bit of trouble getting the housing out, but at a minimum you can get the blower squirrel cage out and jury-rig your own housing for it. Just remember that this is going to have to run off of 12 volt DC, not house current.
To turn the bin into a spray booth, lay it on its side and cut a hole in the bottom, big enough to attach your blower. You’ll need to attach a filter too, which can be done with any HVAC grate, as you would find in a room, perhaps a bathroom, preferably one without adjustable louvers. If you drill some holes in the top side of the bin (as it is laying down) you can use them to attach hooks to hang things from to paint.
The lid of the bin can even serve a useful purpose, covering your spray booth when you’re not using it, so that dust doesn’t get in it.
Cabinet Spray Booth
That small spray booth probably isn’t big enough for most of us. We need something that will allow us to put the projects we are making inside the boot. That generally means a cabinet type spray booth. While cabinet booths can be made literally any size, most are limited to something like five feet long, three feet high and three feet deep. This can either sit on an existing table, or you can make it go down to the floor and set a table inside it. In that case, the opening size would be calculated based upon the space above the table.
While this can be made out of any material you want, it really isn’t necessary to spend a lot of money. Ripping a couple of 2”x 4”s will give you material for the framework and you can cover the sides, back and top with either “visqueen” plastic sheeting or better yet, Coroplast, which is the fluted polypropylene material they use for political yard signs.
The nice thing about using Coroplast for the skins, other than the added durability, is that you can make the sides extend past the edges of your booth, allowing the extra material for the walls and ceiling to become doors. When you open it up, that gives you a larger depth, to help ensure that the overspray gets sucked up by the blower and when you’re not using it, closing those flaps helps keep sawdust out of your spray booth.
The larger the spray booth, the larger the air plenum needs to be. As a general rule of thumb, you don’t want to have more than a foot to 18” of back wall, around the plenum, except in areas where you won’t be spraying, such as under the table. If you have a lot of back wall around the plenum, you’re going to end up with a lot of area where you don’t have much air draw, so the overspray might escape.
Another option to help with your airflow is to split your air filters apart, with about a one foot piece of back wall between them, even though the air plenum is continuous. This will help the air handling system to pull more air from around the edges.
You’ll probably want to mount some sort of light inside your booth as well. In commercial paint booths, that has to be an explosion-proof light. An incandescent light bulb can heat up enough to cause some vapors to ignite. While a CFL doesn’t get that hot, there may be a risk of sparks. To eliminate this, you’re better off installing a LED light. But don’t put one in that has to be screwed into a socket. Rather, get something you can hard-wire in place and solder the connections. That way, you can eliminate the risk of sparks.
Spray Booth Tent
If your woodworking includes making furniture, it would be nice to have a separate room which you could turn into a permanent spray booth. But few of us have room that we can set aside like that. In that case, you might need to set up a temporary spray booth from time to time, which is big enough for spraying entire pieces of furniture.
To start with, you’re going to need a blower, with filters mounted to it, to be your air handling system. Rather than building this with a plenum, you’re going to be better off mounting the air filter right to the opening in the blower. Add some sort of a flange around it, for attaching to the booth wall. Since your temporary spray booth will be outside, you won’t need to have ductwork going into the blower or out from it.
You’ll also need another filter, mounted to a board, to act as your incoming air filter. Put feet on this, so that you can set it on the ground, inserting it into the wall of the spray booth.
The booth itself can be made out of visqueen plastic sheeting and a pop up canopy. These are available in a variety of different sizes and are reasonably priced. Get one with straight legs, rather than slanted, as it will make it easier to attach your walls. You’ll also need a whole bunch of spring clamps.
Spread visqueen on the ground, your patio floor or your driveway, making sure to cover a larger area than what will be covered by the canopy. Then set up the canopy on this plastic, centering it. You’ll want at least a couple of feet of plastic all the way around.
Now attach the plastic to one of the legs, up at the top. Then walk it around the canopy, so that you have plastic all the way around. Cut the plastic off, giving yourself at least a foot of extra material to overlap and ensure that you don’t end up with the plastic being too short, once you clamp it to the canopy’s legs.
Starting from one corner, roll up the edge of the canopy that’s hanging down and the top edge of the plastic together, making a more or less airtight seal, and then clip it together with a spring clamp. Continue working your way around the canopy, doing the same thing at least at the corners and in the middle of each side. Clip the plastic to all of the legs as well, using spring clamps.
Use some scrap 2”x 4”s or other lumber to roll up the extra material at the bottom of the walls and the floor together, making a more or less airtight seal at the floor as well. Once you do that, the only real air entrance you’ll have is the corner where you started and stopped. Tape this with packing tape or duct tape.
Next you’ll need some sort of a door. So make a vertical cut in the plastic, in the middle of one side. This should stop a foot or so below the roof and above the floor. You may need to make a horizontal cut near the floor as well, so as to make it possible to take your furniture inside. Cover that wall with another layer of plastic, attaching it to the corner post at the upwind side. You can temporarily attach it at the other corner, with a spring clamp, once you go inside.
Before being ready to use this spray booth, you’ll need to install your blower into one wall, down at ground level and your incoming air filter in the opposite wall, also down at ground level. Then you’re ready to go, just be sure to clean any sawdust off the piece, before bringing it into this dust-free environment for finishing.