I own a cake air brush machine. I also have a spray booth which has an extractor fan to stop the particles going into my lungs. This is very important. If you don't have one do your spraying outside in the garden. If you have a ceiling fan in the same room as your airbrush, turn it off. The same goes for through breezes, close the doors. This is because fine particles will be air-born and they will land all over your other cakes. ...terribly messy mistake if you don't. This is called "over-spray"
Airbrushing is fun but rather technical, takes a lot of practice and a bit of preparation.
Place your round cake onto a "card-board round" and place it on a "turn out board". Cover the cake in fondant pettinice. Allow to dry for a while not essential but the dryer it is the better. My kitchen has a "dehumidifier machine" so the humidity is not an issue for me. Using a "soft bristle brush" gently brush off any excess icing sugar. (Don't use cornstarch to cover your cakes) Now you are ready to start.
Place the covered cake still on the turnout board onto a "turn-table" in your spray booth with the fan on. On your work bench prepare the mix.
Using "Luster Powder" which is manufactured specifically for cakes and is non-toxic, get a small bottle (film canister, baby food jar, small medicine bottle), and mix up the luster with "pure alcohol 99%" which has 1%rose essence in it. This is so the kids can't spike drinks with it. It has a very strong smell and taste until it totally evaporates off leaving no smell. You can get pure alcohol which has no smell or taste from a pharmacist who trusts you, otherwise you have to get rose alcohol from cake decorating supply places. Do not use Vodka or other dry white spirit as it is still too sticky and will not give you perfect results although it does work to a degree. I mix by feel and then do a test patch.
Load the nozzle of the airbrush head with freshly mixed or if reused, shake the container, of luster mix. Turn the machine on and on a white A4 paper, or dry piece of test pettinice, or broken icing flower, do a test strip. If too watery, add more luster, if the machine clogs or spits lumps, add more alcohol.
Make sure your cake is centered on the turntable as a potter would center clay on a pottery wheel. With your nozzle a fair distance away, and your machine on the finest setting, start spraying the nozzle into the air by the side of the cake. Then move towards the cake starting around the bottom edge first. Make sure it is a clean sweep and the turntable is moving around slowly but gently. This takes a bit of co-ordination as you want to avoid spilling the liquid and wasting it or worse spill it on the cake. To make the liquid come out of the nozzle you just gently pull back on the trigger. Do it slowly. Everything in airbrushing is about slow and gentle movements. The trick is to start off the cake and finish off the cake. If you start and stop on the cake you get spots of concentrated colour and spats. Keep an eye on the pot. If you are about to run out of liquid it may spatter. Keep it filled but not too full as you may spill it.
You can layer colours. The strongest colour is best on the base and the lightest on the top. Imagine a Tequila Sunrise. Each time you go to add a new colour, you need to clean the nozzle. To do this you have a bucket of hot water by your side. Just plop the nozzle in it and keep the air pumping through the machine. That gets the bulk of the colour out. The airbrush manufacturers recommend that you flush the nozzle with a liquid that is a nozzle specific cleaner. I have to admit I never used it. I dried it by shaking it, shine it with a soft cloth, then running the air through to dry it completely. If I had a blockage, I had a paintbrush with a pointy tip and I pushed that into the nozzle head while in the hot water with the air on. I then could put pure alcohol through it. Don't use methylated spirits please. Use the meth to clean the rest of the machine but not the nozzle.
After you finish spraying your cake take it off the turntable. Place it on a drying rack (if in a shop take the tag with it so you don't mix up the cake orders). Prepare your cake presentation board with a blob of royal icing in the middle as glue to stick the cardboard to the baseboard. This isn't necessary if you put a thin layer of icing on the presentation board as it just tends to stick from the moisture in the fondant. When the sprayed cake is dry, using a butter knife around the edge, separate the cake from the turn out board around the edges. Slide your palette knife under the cake and slide the cake onto the presentation board. Place the rest of the tiers on top not forgetting the pillars or internal struts as you go. Just make sure you center the tiers. The turn table is good to check the centering or look at it from the other side of the bench.
Using royal icing tinted to the colour that compliments your cake design, do a shell piping around the edges. If you are placing a ribbon, still pipe but do it right up against the cake and wipe off the excess with a tool and a moistened paint brush tip. Then place the ribbon around the edge. If you did all this work to spray the cake there is no point in using a wide ribbon. Use a narrow ribbon.
NB Troubleshooting lumps: happens when reusing older mix. The bottle you stored it in is too big and has too much oxygen in it allowing some particles to dry inside the bottle. When you shook it you shook those lumps into the mix and polluted it. If you have crusts inside you either throw it out, persist with lumps or pour off the liquid into a new bottle and try and do that without dislodging the lumps. Use a smaller bottle next time. Like an essence bottle.
This is the basics.
From here you can learn to create designs using templates for example a cloud pattern for a christening cake. Then you can layer using different templates ie the leopard pattern great for cake handbags. You can airbrush icing flowers and hand made beach shells. I have some pretty examples on my blog. I am a huge fan of airbrushing and I really pushed the boundaries. I have taught airbrushing techniques.
I am Wedding Cake Enchantress - cake shop owner - on maternity leave