Sunday, February 23, 2014

DIY Cardboard Spray Booth

One of the biggest shocks of moving from the desert to the Midwest is a sudden change in everything outside your home: The air changes temperature, everything gets wet, it gets so not-warm that sometimes you have to dress differently... It's exhausting keeping up with this changing weather. We used to leave the cardboard box we spray-painted things on outside for weeks at a time and the worst that would happen is a small dust storm would move it a few feet. Now, we've been snowed in on the weekends and it simply isn't conducive to spray paint when everything is frozen under a few inches of what I am assured is water that fell from the sky. The sky! We needed something to let us spray paint more-or-less indoors. Maybe you do, too.

You'll Need:
  • A box fan
  • Furnace filters similar in size to your fan
  • A cardboard box at least as big as the fan/filters
  • Duct tape
  • Empty gift cards or other sheet plastic
  • Zip ties
  • Under-cabinet light source
  • Lazy Susan
  • Flexible dryer hose, plus some hardware (hose clamp, some kind of flange for vents)
  • Tools: Cutting tools, straightedge, measuring things, marker

Coincidentally, another big deal in the geek/maker sphere needed a spray booth for his shop, and provided some ideas. Not everyone is in a position to spring for a custom sheet metal one, so I throw this one out there for folks who want a shot at this, too. Comparing the two feels a little like the "Nailed It!" meme on Pinterest... Pressing on!

Dramatic lighting proves we're serious artists.

We sketched out what we were looking for and made our shopping list. I imagine most folks have or have access to an old box fan, so we designed it around that to begin with. Our model is 20" x 20", so all our measurements start from there. We lucked into finding a box 19" x 21.5" from the ones we moved with, and got to work. I knew that I had to accommodate a 4" flange at the back and didn't want corners in the back of the box where fumes could collect. I decided the easiest way to answer this issue was to cut the flaps of the box and taper them all down to a 6" or so plate at the back. Starting with the short side, I did some measuring and figured out the lengths involved, and then transferred those to the larger flap as well. Ignore my math problem in the picture, as I briefly considered making the back plate proportionally smaller than the box dimensions but realized that actually makes the construction way harder and less likely to line up.

The real math is came down to 19" flap - 6" opening = 13", which means 6.5" needed to get cut off either side in two big triangles. Doing the same on the other flaps meant the same angle, which means the same length, which means everything lines up how you want it to, leaving a back opening of 6" x 8.5". Your box and fan combo will be different, so just have a look at the picture and make sure you are doing something similar.

Same measurement, same angle. Ignore the math.

Once cut (being careful to only cut one flap at a time and not slash up the one underneath it), it was time to start taping it all together. My method for taping boxes is to use small squares of duct tape wrapped over the corners to hold everything in place to make sure you like it and then tear big pieces to cover seams. I taped seams inside and out to make sure I sealed up the box as well as I could, ignoring that it is, after all, just a cardboard box.


The plate at the back was made out of the off cuts from the flaps. I figured the back plate was getting most of the stress in this thing, so I built up some rectangles from my flap offcuts and taped them into a rectangle two layers of cardboard thick. Once taped together, I trimmed the rectangle to 6" x 8.5" to fill the back opening. I found center, traced the flange, and carefully cut the three inch circle into the back panel.

Once the hole was in there, I taped in the flange, inside and out, and taped the whole thing into the back of the spray booth.

With that accomplished, it was time to slip the fan and filters into place. It was a little snug, but everyone made it. Make sure your fan faces backwards (i.e. toward the flange in the back) or your wardrobe will be whatever color you're spraying. It defeats the purpose of the spray booth.

The inside of a box is hardly a good place to see what you're doing, so I took the under-cabinet LED light we bought for our last kitchen and wrapped it in clear packing tape to protect it from paint. Testing the light, I found where I wanted it to hang and marked that position on the top of the box. I punched those holes with a knife, marked similar holes on a pair of old gift cards, and holepunched those to match. Slipping zipties through the holes and around the light with the giftcards to reinforce the holes, I put the light in and zipped the ties tight to hold it in place.

The cords from the fan and light were taped securely down the inside of the box and run out to the corner, where they plug into a $4 15-foot extension cord so we can wander with this thing wherever we want to.

The last part is the dryer hose part. Ours was a reduction fitting, so we put the big side inside the box and the smaller side outside so I could use the hose clamp to put our hose on. I slipped the pipe on, tightened the crap out of it, and we had an exhaust port to point where we needed to. The trick here is to find a good place to vent the fumes. I set up by the basement door and hung the hose out the door, so I'm painting inside and the fumes go out there (mostly).

I ran the fan for a little while and found that air was blowing back at me from around the top and bottom, so I built up some cardboard baffles that sealed above and below the fan to keep the air moving away from my work space. Right now they're loosely taped in place, but there may be a more permanent solution to that part shortly in the form of another pyramidal tape-and-cardboard structure like the back.

The two proposed improvements are a slot to slide the filters in so we can by-pass the angled pieces inside, and a plywood plate with a vent hole cut in it that fills the bottom part of an open window so there's no chance of the fumes blowing back in through a doorway. We'll see how many improvements we can cram into a cardboard box before I break down and build something for real...


  1. I know this is an older post, but thank you. This is very detailed and just what I was looking for. I would need a few modifications for my purpose of sealing projects with varnish but this is by far the best I've seen. Thanks again

  2. Thank you as well! I used your concept and basic build instructionsomething to get on of these put together.

  3. This is a good concept but, there is not enough space between the vent hole and the box fan. This design causes some blow back. I am currently trying different configurations to eliminate the blow back.