Introduction: Benchtop Spray Booth
I make wooden shoe shaped objet d'art about 300mm x 400mm x 100mm. I need to spray paint these objects, which at the moment means outside in the front of my house. There's a few reasons why that is not a long term plan? Firstly winter has just started and below freezing temperatures are not conducive to spray painting! Secondly, it's not a good look for the neighbourhood to witness me releasing airborne contaminants into the air!
The next logical solution was therefore to spray within my garage/workshop without any detrimental health concerns. Therefore a spray booth mounted on my work bench was contrived. A quick google of manufactured booths showed that these aren't cheap and way outside my budget. The alternative of not using a booth means solvents will quickly fill the room with fumes and overspray fog. In theory, any spark-producing device, such as a switch, light, or radio, could trigger a solvent explosion? This applies equally to using an exhaust fan incorporated or exhausting from a booth. If your electric motor powered exhaust fan is exposed to flammable contaminated air and is not flameproof, then you're at risk! This includes axial type fans used to ventilate bathrooms. My 2hp dust extractor has a motor outside of the air stream and doesn't use the moving air within the fan to cool the motor. The blades of the fan are however metal and theoretically if the metal fan blade was to somehow strike the inside metal fan casing, then you might have an ignition source? Therefore I recommend that you only exhaust if the motor is outside the airstream! Another factor to consider is you need to exhaust air at a velocity of 0.5 m/s or 100 cfm measured at the filter inlet. Not enough velocity and you risk building up an explosive amount of vapour within the room. Too much velocity and the filter might not function properly to capture the paint? I resolved that I would use my sawdust extractor fan as the exhaust, but disconnect the blower discharge duct from the filter and duct it outside through an open window. If you only use water based paints then you're laughing?
Disclaimer: This Instructable does NOT comply with government authority standards for a flameproof ventilation system! Proceed at your own risk as I take no responsibility for the consequences?
This spray booth is bigger than those cheap and nasty airbrush booths. The design needed to be useful for not only spray painting but other purposes:
1/ As an exhaust hood behind a compound mitre saw, that sprays dust in all directions.
2/ As a bench top exhaust when hand sanding with an electric sander
3/ As a photographic studio for still images of finished products! Yes with a backdrop sheet, the booth LED lighting will make for a controlled lighting environment to take stills.
Another requirement was a sensible final weight (around 20kg)? Not too heavy to carry and lift on/off the bench top. Also that it was compact andfoldable in some way to minimise the storage footprint?
I got busy using masking tape on the surface of my work bench as an outline. I have a sawtable fence that I don't want to have to remove everytime I use this booth, so it needed to sit over it somehow? After 3 different prototype designs using Fusion 360, I came up with this design. It is constructed from 12mm MDF. The filter plenum is 600 wide and 900mm high. With the hinged wing and hood lid pieces unfolded, the opening is 900mm x 900mm x 400mm deep to the front of the filter.
2 off 1800mm x 600mm x 12mm MDF sheets and 1 off 1200mm x 600mm.
2 off 900mm long and 1 off 600mm long piano hinges
1off 64mm steel stud bottom track as used in house frame construction
1off 40 x 40mm light guage zincanneal angle as used for internal corner reinforcement of plasterboard walls
6mm dia x 38mm long hardwood dowels
M5 x 38mm Confirmat screws
M5 all thread and wing nuts. cut and epoxy 40mm long all thread studs into top plenum edge, with wing nuts for quick removal of the lid. Adhere rubber gasket to edges.
Shellac/Oil based primer and top coat paint
A 600mm long LED overhead light and 2 LED sidelights
Spray paint filter: Andreae Exhaust Concertina Cardboard 900mm high x 9m long
100mm dia spigot for connection to flexible ducting
600mm wide white roller blind
3mm pop rivets
5mm x 40mm bolts and wing nuts to fasten lights and roller blind.
Step 1: Construction
The tools required will be:
Table saw, and/or a track saw. Even a hand held circular saw will do it. A handsaw will suffice as the worst alternative. Hack saw
A drill, preferably a bench press. There are a lot of dowel and confirmat screw holes to drill, so plumb accurate drilling is necessary. Use of a dowel jig will do the job though?
A roofing square for mark out
Phillips tip screwdriver
Drill bits, about 80% of the dia of the screws
LH & RH Tin snips for cutting the filter frame
I set about marking the MDF sheets and cutting out all the pieces. Please use a dust mask. Because the two side panels of the plenum need to be bevelled where the hinge affixes, it's best to over cut the width by say 20mm. Then drill and glue in dowels for hinge screws. Then do the bevel cut, cutting through the dowels along the edge. The bevel angle was 21 degrees. I used a track saw for this. Now that the bevel is cut, measure across the width required, mark and cut the other side. As I proceed I rub PVA glue with a cloth along all cut edges. This helps seal the MDF prior to paint application. MDF is notoriously absorbent on cut edges and you can spend ages sanding and repainting "furry" edges. The glue seals the edges quickly. Once everything is cut out and dowels installed, I mask the edges that will be a glued butt joint to another panel. Glue doesn't stick well to painted surfaces. Next, mark out and drill the confirmat screw holes in the edges. As well mark & drill the faces for the matching holes. These holes need to be the dia of the confirmat screw outer shank dia. Then using a countersunk bit, ream out the tapered screw head holes. Time to paint everything. Once dry screw on the piano hinges.
Next step is to build the metal filter frame. This comprises a metal "C" channel as the base/head and "L" section metal corner angles for each side with a rectangular size of 600mm wide by 900mm high. The top "C" section will be fixed to the underside of the MDF lid. This lid is removable by unscrewing 6 all thread studs to allow access to the filter for replacement purposes. Pop rivet the bottom corner frame junctions, as well as pop rivet metal to mdf.
With a stanley knife cut a length of cardboard filter off to 600mm expanded width. The issue now is how to retain the pleated concertina filter in the fully open position? I made 6 clips using 50mm widths of the "C" channel. I hammered over to flatten one wing of the channel. The other wing got hammered over too, but not fully to leave a open bevel that hooks over the cardboard pleat. The hammered flat side of the clip gets pushed between the metal angle side and mdf in between fasteners. there's enough play for them to slide in and out. last step is to fix the lights and roller blind. I used 5mm bolts and wing nuts to fasten. When you want to fold the booth up, you'll need to remove these fittings quickly.
Step 2: Final Testing
I hooked a 100mm flexible duct from my 2hp blower to the rear of the plenum. Then 125mm dia discharge duct from blower through a window. They say you need 0.5m/s velocity of air at the filter face? I was able to measure this and I wasn't getting that at all. Felt like I was getting nothing? I was instantly disappointed, and thought, "I've gone to all this work for nothing!"
At the inlet to the 100mm plenum duct I was getting around 14 m/s but the 900mm x 600mm plenum dissipates this velocity! However, if I held up a sheet of newspaper 300mm out in front of the filter, it was being pulled towards it and would adhere to the filter if I let the newspaper go. I guess the 0.5m/s is specified for a room sized spray booth for painting a car. It needs to drag spray from 1.5-2m away from the filter. Maybe I was being a bit harsh on myself?
I built a lazy-susan object platform that raised the underside of my subject object up about 200mm. This meant that a spray can or gun would always be plumb. Having the lazy-susan to rotate my object meant I was always spraying directly at the centre of the filter. This really improved the paint finish with even application, minimal runs and no overspray. The blower dragged all the contaminated fumes away form my face and left the garage not filled with spray vapour at all. Overall I was very pleased, and the whole project came in at under $200* (*Filter costs about $100, but you'll get about 15 spares from one box, so a unit cost of only $7). As a sanding booth and light box, it worked a treat too! Follow along on my creative endeavours at: instagram.com/foreverandaday11