Spray Booth for Small Shops

Awhile back I embraced HVLP spray finishing, and I’m sure glad I did. I built a 200-square-foot shed in the summer of 2015, and sprayed the interior walls white for a bright, clean look. Cringing at the thought of brushing and rolling seemingly acres of protruding lumber and recessed plywood cladding, I took the plunge I’d contemplated often and bought an HVLP paint spray system. Wow, what a great way to apply finish!

Since then, I’ve enjoyed spray finishing furniture projects in my shop using a makeshift portable booth setup to draw away the little overspray HVLP creates. After the HVLP kit, a basic booth is the best friend you can have when spraying indoors; and in Canada, that’s most of the year. My makeshift booth, a large cardboard box cut and folded to form an alcove, with a furnace filter and window fan set in the rear to draw and filter air, was fussy to set up; floppy, requiring bracing, and it proved too small for anything but table-top projects. The concept worked fine but it needed refinement.

To be clear, this design is only for use using waterborne finishes, never for spraying finishes with flammable, potentially explosive solvents.

Simple and effective

Here’s my redesign; light and portable, always fast and easy to set up, it’s compact to store when not in use and modular, so I can set up as much or as little as I need. It’s very budget friendly and made from readily available materials. The concept can be scaled up or down to fit anyone’s needs, project size and available space.

The structural bones of the booth are 2' × 8' × 1/2" sheets of closed-cell foam insulation and 1-1/2"-thick clear pine frames that were milled and grooved to accept the foam. Two furnace filters, solid wood to create a cavity to secure the furnace filters, 2-1/2" loose pin hinges, a tube of translucent caulking and some 2" flathead wood screws to draw the mitred corners together are the only other materials needed.

I used 20" × 20" filters, as that’s what my home furnace uses, but using 24" filters would simplify the construction of the two center frames. Alternately, you could adjust the width of the frames to accommodate the same size filter you already use at home.

The full setup is six 24" × 72" panels, but for most finishing projects I’ll use only the central two or four panels, leaving the two “wings” on the shelf. Regardless of whether I’m spraying a small box, a chair or a cabinet, I can form an alcove with plenty of airflow and filtration behind any project so overspray can be drawn away effectively for a perfect result.

Furnace filters

The two centre panels are each made to hold 20"x 20" disposable furnace filters 32" above the floor, so one or two 20" window fans can sit fully flush to the filters. Typically, the fans(s) rest on a bench or other work surface behind the booth while the project being sprayed is supported on a versatile wheeled shop table, but this can be altered easily to suit my needs.

Construct the frames

The framed panels are joined by hinges with ‘loose pins’. By accurately marking, mortising and insetting, the hinge halves
can be joined or separated at will; panels can be inserted, added, removed or interchanged readily. The gap between panels is never greater than 1/8", easily bridged by painter’s tape if preferred.
Beginning by jointing and milling my stock to 1-1/4" thick- ness, I then ripped 1" wide strips to make the frame material and machined a 1/2" × 1/2" groove in one face. As the foam sheets are never uniformly 1/2" thick, some finesse passes then tweaked the width of the grooves to firmly slip fit the individual panels.
With the foam sheets cut to length, the grooved framing material was mitred to snuggly border the foam, leaving 1/16" room for final fit. Once dry-fit flat on my bench, I drew a sight line 45° across each corner, an eyeball reference to drill two countersunk holes across each mitre, centered 7/16" in from each frame face. I then sunk two 2" screws to bridge and unite the frame members at each corner.

Groove Frame Pieces – Salusbury routed grooves to capture the insulation in the face of the pine frame stock. This operation could also be done on the table saw.

Mitred Cuts – A sacrificial fence on the mitre saw keeps the cut crisp and reduces the chances of airborne offcuts. Salusbury used a stop to ensure all like frame members were the same length.

Secure With Screws – Two screws installed in countersunk pilot holes keeps the corners together. Notice the pencil line at a 45° angle; it helps guide freehand drilling.

Install the hinges for your spray booth

With all six panels assembled and screwed together, I sorted out which would be the centre, left-hand and right- hand pairs, pencil marking each panel to identify bottom, and mating sides for order of assembly. This showed me where to apply hinges, being sure to install them pinhead up. Referencing from the bottom only, I mated, aligned and clamped each pair of panels, then marked each hinge location using a layout knife and machinist’s square, bridging both panels for continuous layout across the assembly.

I repeated this process for each of the five mating edges. Next, with the panels paired, I routed out the waste within each hinge mortise, tuned the fit with a keen chisel, inset the hinges and using a self-centering drill bit drilled out the screw holes then installed the screws. With a pair of panels now united by hinges, I tapped out the loose pins and moved on to the next mating edges, repeating the process for all five mating edges.

Large or Small Booth – Precisely installed hinges assure interchangeability of booth sections down the road. The hinge pins are easily removed so the user can use all, or just a few, of the booth panels.

Furnace filter cavities

Next, I focused on the centre pair of assemblies, creating
the openings to receive the filters. I began by making framing material, some 1-1/4" × 15/16" with a 1/2" × 1/2" rabbet in one edge. The foam panels would fit into the 1/2" × 1/2" rabbet. I also machined some 7/16" × 1-3/4" flat stock to frame the backside of the foam while providing a 1/2"-wide backstop for the filters. With this trim dimension, plus the outside measurement of the filter, I used a sharp knife to cut an opening into the foam, cut the L-shaped filter trim to length so it would fit between the outer vertical frame members and then installed the horizontal filter frame using Titebond glue and a few pins. I also attached the flat stock to the ‘L’-profiled inner frame/ border to give the filter something to stop against. Considering differences between filter brand sizing, I allowed a 1/4" for a loose filter fit (1/8" all around), the filter retained with blue painter’s tape from the ‘working’ side.

Filter Use – This cross-section shows
the cut-away foam insulation panel fitting into the L-grooved horizontal filter piece, as well as how the added piece on the right helps hold the filter in place. Blue tape is the final addition to ensure the filter doesn’t fall out during use, yet is easily removed for replacement.

Joinery Details – Salusbury butt joined the vertical filter frame member in place with glue and pins. It has a groove on its edge to accept the narrow strip of rigid foam insulation. A narrow piece of wood was also attached to the opposite side of the frame, on the edge of the outer frame member, for the insulation to butt up against.

Caulk the corners of your spray booth

With all the panels completed, I did a final check for square and straightness. That done, I secured all the elements with
a tiny bead of translucent interior/exterior caulking spread everywhere the foam contacted the frame. Once cured, it pro- vides a unifying bond yet will allow me to easily cut it away if I ever want to replace the foam.

In use, I use one or both central panels, as well as up to four side panels, depending on the size of the project and the volume of fluid I expect to be spraying. Simply removing and installing hinge pins is all it takes to set up as much or as little as required.

A permanent improvement over a floppy cardboard box!

I went for cost effectiveness, lightness and versatility using pine and foam, but this proven concept can be adapted. The foam could be substituted for stiff cardboard, or 1/4" plywood. What will you make for your shop?

Ready for Action – When only a small project is being sprayed Salusbury sets up just three panels. Larger projects require all the panels to be assembled.

Light and Stackable – The broken-down spray booth stores flat on a shelf, waiting for the next project. The panels are very light, and can be handled above head easily.