Spraybooth

I set out to build a spraybooth so that we could paint our models using aerosols and airbrush. The aim was to build a booth sufficient to spray small models, up to about the size of a 1/72" Lancaster, an HO gauge coach or similar. We decided to build a central turntable on which to place the models during spraying, to allow them to be sprayed all round without touching the paint.

The plan was to use materials lying around in the workshop. This included most of a sheet of 12mm ply, a half-sheet of 8mm MDF, a decent-sized offcut of 18mm ply and some odds and ends some of which were recovered from previous dismantled things. The fans and lights were new, the 3A plug and socket were from the spares bin as was the cable, Perspex, plastic angle, aluminium angle, handles and most of the bearings and other components used in mounting the turntable.

Advice and hints were gratefuly received from the Sheddi (nntp://uk.rec.sheds). This included: use lights, use a door that can be shut to allow the models to dry without getting dust on them. I also checked some sites on the web to find out about types of spray booth, and that's where I picked up the idea of downdraught extraction.

A comment: apparently (actually knowingly) this construction is dangerous; the inline fans are not rated against sparks and consequent explosion risk. It lives in an outbuilding, the filters remove most of the paint, the remaining solvent is venting to outside air, and any ignition will result in a flame out of the exhausts. It hasn't happened yet. When I can afford the right type of fans (with the motor out of the airstream), of course I will buy them.

Main construction
For the main body, I decided to use 12mm plywood, glued and screwed with reinforcements as necessary, working to a cubic plan of approximately 24" on a side. The front face was set back by 8" at the top, to allow easier spraying downwards onto the top. Lights were mounted in the top. Air is drawn out of the bottom through two 4" duct couplers. Use good ones, the cheap ones are very thin and break or distort too easily.

I decided on 4" inline fans, as being cheap and readily available. Past experience indicates that the axial-flow models are generally poor, so I went for the larger but more expensive radial flow. These give a flow rate of about 30l/min, higher than the axial flow fans, and they also have a longer life. I looked at some sites on spray boots and air volumes, and guessed that two fans would be necessary. I validated this when I had built the spray booth (but not the fan box), airflow was more than adequate, allowing me to include omre filtering than I had originally anticipated. My original plan was to use plywood baffles to remove the paint, but the paint mist is so fine that I decided to fit filters. I experimented with Dyson filters (which fit the 4" ducting) but these were not at all effective (hence the black spray visibleon some of the white components in the pictures), so I tried car air filters, and these work very well. I am using two sets of filters, one would probably be sufficient, very little if any paint made it past the first filter in tests.

The first stage in the filtration process is the support box, which has a curtain of 6mm ply round the edge. I was going to match this to another box of 6mm ply mounted on the bottom of the spray booth, to give interlocking channels and hopefully start the process of slowing the air and dropping the paint. I may still do this. I also fitted a ring-type air filter (can't remember which car, but it was the cheapest ring filter I could find). This goes round the two 4" couplers to the fan box (plain 4" holes in the bottom of the spray booth, the couplers fit to the fan box). I covered the whole with an oval of 12mm ply, with two screws to retain and seal, see fig. 7. Be very gentle, air filters crush easily. The bends in this one are the result of me kneeling on it by mistake. Bah!

The doors are made of 8mm MDF and hinged with piano hinge, fixed with 3mm countersunk setscrews and Nyloc nuts. You could use cardboard with duct tape hinges.

Turntable
The turntable is about 18" in diameter and made of a single piece of 18mm ply.

For the turntable support I thought a clutch bearing would be a good choice but also that the edge of the turntable should be supported. Having in the spares box a set of eight sealed bearings, of 6mm outside diameter, I cut some short lengths of steel channel and bolted the earings to the inner surface. I tidied the lengths of channel with adjustable spanners (good for bending metal where you want), files and bench grinder. In figure 6 you can see the bototm of the turntable, with the supports mounted.

The clutch bearing was from a local mechanic. It's not new, but it works smoothly enough. My friends at RA Engineering gave me a roller sleeve bearing as well; this is mounted in the centre, held in by plastic pipe slotted so it could be shrunk to fit inside the clutch bearing. This should ensure that the pin which goes down the centre (which is plastic) does not wear. The centre pin is 20mm plastic conduit in a disc made of 12mm and 18mm ply. There is not much force on this, it only needs to hold things together.

I made brackets to mount the clutch bearing, the holes are slotted vertically to allow the angle and height of the bearing to be adjusted. When assembling I placed the turntable upside down on the bench, packed with thin card round the edge supports, placed the support box in place, moved the main bearing firmly into place and tightened the bolts. This slight preload on the main bearing means that in use the edge bearings barely touch the support box. The turntable turns remarkably freely, but the clutch bearing has a decent amount of drag so it does not spin. Too light a turning action may result in the model being blown round by the force of the spray.

Lights
The lights are standard 12" interconnecting fluorescent lights sold for kitchens. I used five, three would probably be sufficient (you can get 15" ones), or indeed a clear top to the booth. To protect them I used a sheet of Perspex with a piece of plastic angle glued on (using solvent weld waste pipe adhesive) to stop the front edge bowing and to make it easier to get in and out. It's covered with cling film in case of overspray. It sits in a groove made of aluminium angle assembled with 2" x 1" planed timber into which a shallow groove was routed so that the wood clamps the aluminium and sits flush to the side of the box. Screws through both wood and aluminium hold the assembly in place. A switch was placed on the side of the spray booth, and a cable with a 3A plug fitted; this plugs into a socket on the fan box so only one plug is required into the mains supply.

Pictures 8-12 show the lights and the perspex cover.

Fan box
The fans are the key to the whole thing: you need to ensure smooth and adequate airflow, without the fans becoming covered with paint. They are not designed for that. Ideally you should use explosion rated fans, since the overspray is likely to be loaded with highly flammable solvent. I could not find any. I hope we'll be OK. Obviously the more paint and aerosol solvent you can get out of the airflow before it hits the fans, the better, hence the air filters. The exhaust should be ducted away. That reduces the airflow. One fan will probably not be enough. The plan for us is to fit two cooker extraction outlets through the garage walls, so we can vent to outside.

For storage I thought it would be best of the fan box fitted inside the spray booth. It's heavy, so i fitted handles to the spray booth. I routed a shoulder into the 4" holes for the couplers to the spray booth, because they have shoulders on them and would otherwise sit proud.

Air comes into a central chamber (you can see this in figs. 18 and 19 below). From here it passes through an air filter into a void, and is sucked out of there through two short lengths of 4" duct into the fans themselves, then out to the vent pipes. The white tubes you see poking out of the back in fogs. 20 and 21 are the fans themselves. The bottom of the centre chamber slides into place so that the filter can be changed. The only air selaants used are PVA glue around the joints, air leakage is not a big problem judging from the airflow.

Tests
I undercoated the inside of the booth white, to ensure that wood dust did not fall on the models. You can see from the pictures that some overspray hits the sides, but that very little goes up. I sprayed with the lid half down, using chaos black, a large number of Games Workshop mini figures. The results were very good, although placing them on a box to make it easier to get paint cover to the underside of chins and bodies may improve it further. There were no obvious areas left unpainted. I shut the lid and turned the fans off to allow the figures to dry without attracting dust.

While spraying, it is very noticeable that the workpiece remains very clearly visible as the overspray is removed with remarkable efficiency. It's not like spraying paint into a hoover, but the air flow may be a bit excessive for efficient paint usage. I wanted to control the fan speed with dimmers, but this does not work on these fans. I will add a second switch so I can switch them independently and try with a single fan.

Overall, though, this is a massive improvement over the old faithful TV box with a hole cut in the side. Paint coverage is even and solid and the model can be turned to get it even all over. Best of all, you don't get covered in paint yourself, and you can see what you're doing. It cost more than I'd hoped (fans are expensive) but is worth the money and time invested.