I had bought some powder coating powder from electrostaticMAGIC a while ago, on the offchance I could do something with it. I eventually got around to rigging up a simple electrostatic spray gun, using an air blowgun, a jamjar, and my trusty flyback power supply. Here's the results.
Please note, I don't intend to detract from the products sold by electrostaticMAGIC, by showing how to make a coating gun. Mine is rough, a bit unreliable, and assumes you already have a HV supply to use. I mainly wanted to make a gun for doing very small parts, since their gun is a bit on the large side. electrostaticMAGIC do a complete starter kit including gun, HV supply, containers, and a bag of powder for only £138 inc. VAT (last I looked) - you'd be well advised buying it, if you want to try powder coating!
The principle is very simple. A low-pressure air supply (around 10psi) is blown through the bottom of the jamjar using the blue pipe. This stirs up the powder, and expels a cloud of it through the nozzle. Insize the nozzle, there's several pins which are connected to the HV supply - these charge the powder particles as they leave. Baking the finished article for 10min at 180°C seemed to work pretty well for fusing the powder.
It is a fantastic finish, and I'll definitely be using it on more things I make (e.g. metal project boxes, a favourite for powder-coating). You can get an amazing range of colours, including transparent, metallic, sparkly, hammered, and two-tone. I sandblasted the surface of the metal first to provide a really good key for the powder.
Video of coating: