Tuesday, November 21, 2006

AV - The Air Brush Trick


It's not much of a trick. But then, it's not much of an air brush. $4.99 from Harbor Freight. #6131. Little jobbie made in Taiwan, but not very well. Made, that is. Oh, it'll blow paint. Any airbrush will blow paint.

Air brush is a syphon-type spray gun. The paint is in a little screw-top jar that plugs into the gun. The gun is about the size of a ball-point pen. The jar has a hole in the lid so the thing is open to the atmosphere. The jar hangs under the nozzle of the gun. To make it blow you press a little button on the gun. Air blows across a tiny orifice positioned in the throat of a venturi. The orifice is actually a needle valve but we'll get to that in a minute. A suck-up tube runs from the orifice down into the jar of paint. When Mr. Bernoulli reduces the pressure on the venturi-end of the suck-up tube, atmospheric pressure pushes the paint up the tube and out the orifice, where the stream of air blowing past atomizes the paint and blows it out the end of the gun. So mebbe we should call it a push-up tube.

There's not much in the way of control. The paint comes out in a circular pattern, about the same as you'd get from a spray can. And while you can't control the shape of the pattern, you can control the size of the pattern. You can also control how much paint you deliver by adjusting the air pressure. Plus, you can thin down a thick paint, lay it on in thinner coats. Or leave it thick. Air brush don't care. Thick stuff just takes more air pressure. And comes out in bigger particles. Air brush will spray molasses if that's what you want to do.

So there you are with this itty-bitty spray gun. Jar only holds two ounces. If you're an artist, you buy a lot of jars, fill them with different colors of paint; Keep on Trucking and all that sort of thing. To spray a different color you unscrew one, screw on another, after flushing the suck-up tube in a jar of thinner in between. Just open up the needle valve to flush it out good.

Of course, two ounces isn't a very useful amount, not if you're building airplanes. But just as artists use different bottles, so can you. Larger ones.

I use gravy jars. Heinz. Usta have 57 Varieties. Gravy jar holds about six ounces. I tried Spanish olive jars for a while but they didn't have the right wrist action. Some buys use baby food jars but I've become a confirmed Gravy Jar Man. You can't use a really fat jar because the air hose screws on to the bottom of the gun and gets in the way. But you can use a tall skinny jar, which is what I did during my Spanish Olive period.


Got a 7mm deep-socket? Take the air brush jar apart. No, not the gun, the JAR. You'll have to pull the suck-up tube off the fitting. If it's stuck, heat it. Don't damage it, you'll need it later. Once the tube is out of the way use your 7mm deep-socket to loosen the nut holding the suction tube fitting to the lid. Take it apart and keep track of the washers.

Got a Unibit? A Unibit makes drilling holes in thin stock a breeze. Your new lid is thin stock. Drill a pilot hole for the Unibit then open it up to accept the air brush fitting. It's about 6mm in diameter, which is close to 1/4" so try that.

While you're drilling, put a #60 hole in the lid out near one edge. That's your vent, the thing that lets those fourteen point seven pee-ess-eyes push the paint up the suck-up tube.

Now put your new lid together. Transfer the hardware from the air brush lid to your Gravy Jar lid. Make sure the vent hole is at right angles to the outlet of the suck-up tube. If you don't, you'll end up spilling paint all over yourself. (At 90 degrees from the axis of the spray gun, the vent hole will always be near the apex of any tilt you put on the bottle. If you position the hole anywhere else, there will be some tilt-angles you can't use.)

Your bigger jar will need a longer suck-up tube so go punch a hole in a used spray paint can. Let the residual propellant escape then cut the bottom off the can. Inside you'll find a free marble (!) and a length of suck-up hose about 7" long. That's too long for your Gravy Jar but you can cut it down. Except it's also too big around - it won't fit on the suction fitting in the lid. Metric vs whatever and all that. So go find the original air brush suction tube. Snip off about half an inch and slide it onto the suction fitting. Now slide the spray can tube over the smaller air brush tube. It won't want to go but it will, if you heat the larger tube and put a little muscle into it when you slide it on.

Why the tube out of a spray can? Because you need tubing that will withstand MEK and toluene and lacquer thinner and whatever else you might use. You can buy such tubing but the minimum quantity is about five feet and it costs the earth. And there you are, with half a dozen useful lengths of the stuff inside old spray cans. Plus, you get a free marble.

So how long does it have to be? It should reach the bottom of your Gravy Jar. Not smack up against it, but pretty near. You'll also need several extra jars plus one extra lid. (Why an extra lid? Because you've just converted one of them to your sprayer-lid.)

That takes care of making your new lid, which is 90% of the job. But like everything in homebuilding, the remaining 10% of the job will take 90% of the time.


See that crappy little black plastic air hose that came with the air brush kit? It's going to pull off its fittings about ten minutes after you start using the thing. So go find some fine stainless steel safety wire, put a couple of warps around the fittings and twist them tight. Po' Boy hose clamp. Works, too. Snip off the twist and tuck the end flat. A bit of tape over the safety wire will keep you from cutting yourself. (And yes, that does happen to be a Band-Aid on my airbrush hose. No, I don't want to talk about it.)

The air brush kit comes with a fitting that connects to cans of compressed air (!) Seriously. They sell canned air at hobby shops and the like. They don't hold much air but if you're painting toy trains or model airplanes, you don't need much. For painting real airplanes, on the other hand, you need a buncha air. Mebbe two bunches, before you get it all done.

To provide air to the spray gun you need to buy an adapter that will mate with the fitting on the hose in the kit. Harbor Freight sells one, #P-1655. It's got a male thread on one end to match the fittings on the air brush, and a female 1/4" NPT on the other. To make it match my air line I added a quick connect. Harbor Freight sells those too but I don't know the number.

The 1/4" airline adapter (P-1655) allows you to use your air compressor. Or any other source of compressed air that has a 1/4" NPT fitting hanging off the end. Such as a big tank of air for refilling a tire. Which will blow more paint than one of those itty bitty cans from the hobby shop.

The air brush is sensitive to air pressure. If using canned air, the lid has an adjuster built in. But when using an air compressor, air tank or whatever, you'll need to get yourself some form of pressure control in the line. Most of the time you'll only need five or ten pounds but with thick stuff you will need as much as 90 psi. Because the gun is easy to adjust you can get by with a very simple restriction-type ‘regulator.' (It doesn't do much regulating but it does cut down the pressure.)


That depends on your financial situation. Sixteen ounce rattle can of ZC primer only has about seven ounces of paint. That makes it pretty expensive. Plus, the nozzle is always clogging up and every time you want to use yellow all you got is green or visa versa.

Quart of primer contains a quart of primer. Cut it 1:1 with reducer, you got two quarts. That's sixty-four fluid ounces and that works out to about one eighth the cost of using rattle cans.

And then there is other stuff to paint, such as steel parts, which get a baked coat of primer then a baked coat of gloss black or whatever. Having a little spray gun that's easy to set up and simple to clean makes blowing a little paint quick and inexpensive. It takes about a minute per rib to give them a coat of ZC with the air brush.


At five bucks a copy you can be pretty sure your airbrush won't outlast the Pyramids. And it is going to accumulate some wear. It's got a couple of teenie tiny O-rings that probably won't cost more than ten dollars each, plus the hernia you'll get from digging through the McMaster-Carr catalog trying to find them.

So buy two kits. The second is to provide spares for the first. At five bucks a pop it's the logical way to go.


Get yourself a box about a foot on a side. Cardboard is okay. Give it a coat of paint; mebbe stiffen it up with some stringers. Use it to store your Air Brush Kit. Keep the thinner and spare jars and spare parts and stuff like that in the box, along with your pipe cleaners.

One of your jars should be filled with thinner. Lacquer thinner will do to clean up after most primers. The other jars are filled with whatever you want to spray, from kerosene to zinc chromate. To give the glass jars a bit of protection, wrap them with several turns of old fashioned friction tape – the rubberized cloth stuff.

Store your pre-mixed primer, tightly sealed, in the refrigerator. If you've got kids in the house, store the stuff in an ammo can out in the shop. Keep it on the concrete deck, covered by several layers of cardboard to keep the stuff cool.

Every time you use your spray gun you gotta clean it. Takes mebbe five minutes, tops. Here's how.

Unscrew the paint jar, put the lid on it and put it back in the refrigerator. Plug your sprayer lid into your jar of thinner and blow thinner into a rag or can until the suck-up tube and nozzle are clear. Get a little thinner on a rag and find your pipe cleaners. Dip a pipe cleaner in thinner and have it ready.

Shut off the air, disconnect the hose and put it away in the storage box.

Take a look at the gun. See that little circlip? Open up the screwdriver blade on your jackknife and pry off the circlip. Now you can push the brass needle valve down through the body of the gun. Unscrew the needle valve as you go. The cone of the valve stays on this side, only the needle part pushes down & out of the gun. Unscrew the nozzle.

That's it. Three parts. Four, if you include the circlip.

Use your pipe cleaner to ream out the needle valve and the nozzle. Get ALL the paint outta there. Use the rag to wipe any paint off the suck-up tube and sprayer lid.

Once everything is clean, put it back together and store it in the box.


The last part of your spray-paint kit is the spray booth, which looks suspiciously like a cardboard box. Hang it or nail it near to a window and rig some dryer ducting from the box to a panel in the window – you want those fumes to end up in the neighbor's air conditioner intake, not in your shop. Rig a small fan to pump air out of the booth and into the ducting. In case you hadn't noticed, a fan is an air pump. To make it work like a pump wire & tape it to the wall of your spray booth. On the outlet side, rig a plenum chamber (which also looks suspiciously like a cardboard box) and fasten your ducting to the plenum chamber.

Making a spray booth is one of the few times in your life when you can use duct tape on a duct. Enjoy.

You don't need a very big fan because you aren't using a very big spray gun. The boxer fan out of a computer power supply will work but most of them run on 12vdc and are noisy. I use a cheap (cheap!!) import desk fan, about six inches in diameter.

To make cardboard useful it needs to be stiffened with wooden stringers. Lath will do fine. Use urethane glue and staples. Once the glue cures, give the box a coat of white house paint and repaint the interior periodically, not only to renew the white surface but to bind up any over-spray residue. After you finish your plane, cut the box up and get rid of it.

When adding stiffeners to your spray booth, be sure add a couple to the inside, up near the top of the box. Rig a couple of wires up there. To paint stuff you'll typically hang it form hooks of bailing wire and use a wand of the same stuff to hold the part in position while you shoot it with paint. Most primers dry in a matter of minutes but it's a good idea to rig a curtain on the front of the box so you can leave parts hang while they dry. In a cold or damp climate, adding a door and a couple of light bulbs NEAR THE BOTTOM will turn your spray booth into a drying booth.

The spray booth is to keep from blowing toxic residue all over your shop. Everyone worries about hexivalent chromium but the truth is, alkyd and epoxy resins, when in aerosols, are just as bad for you, although for different reasons. So even though you have a spray booth that passes your problems on to the neighbors, any time you blow paint you should dress for the occasion. That means an air mask or industrial quality respirator, and gloves that are impermeable to the thinner & vehicle in whatever paint you're spraying. (And this applies to rattle cans as well.)

Most volatile vapors are explosive. Try not to weld while painting, nor to paint while welding. And the smoking lamp should be out.


Back when Tony Bingelis was painting his Falco one of his neighbors complained about the smell. I told him he didn't have a big enough cat box.

The average cat box is 20" long by 14" wide. So you build a box 20-1/2" long by 14-1/2" wide by about 30" tall. The box slides down over the cat box, which is four inches deep and contains 2" of water. Inside the box you've built, there are five partitions spaced about 3" part, except for the input and output which are about 4". Two of the partitions are open at the top but extend down INTO the water. The other three partitions are sealed at the top but only extend downward to within one inch of the water.

Any place you need to form an air seal, such as along the edges of the plastic cat box, cut up some urethane foam, give the surface a spritz of upholstery glue and stick it to it.

On the output of your cat box you put a large fan positioned to suck air through the box. The inlet is on the top of the box at the far end.

What you've just built is a kinetic air filter. Each time the air is forced to reverse direction, which it must do to negotiate the partitions, heavier particles will tend to collide with the surface of the water and stick there.

Even a one cat box unit is surprisingly effective at eliminating odors and trapping paint particles. But if one unit isn't enough, add another to either end. When combined with a labyrinth filter at the input (ie, a stack of furnace filters) and an electrostatic particle precipitator on the output, you can get clean room conditions with only seven reversals (ie, a two cat-box unit).

If you rig such a unit for your spray booth the odds are your neighbors will never know when you decide to blow a little paint, since none of it – nor any of its vapors – will get out of your cat box.

The Cat Box Principle also applies when painting the entire airplane. Rig an air dam across the garage so the door comes down onto the dam and install the cat box in the middle. Seal up any gaps with cardboard, foam and duct tape. To get a flow of air into the shop, use a window or door on the opposite side of the shop. Build a light frame of wood to fill the doorway or window. Glue furnace filters to the frame. Put the frame in place any time you want to blow paint. It won't stop a sand storm but it'll keep out the bugs and most dust.


So you go down to the local EAA chapter and tell the folks about your nifty little air brush and your cardboard spray booth and your cat box air filter... and everyone will tell you what a bad idea it is. Spray cans are better. Zinc Chromate is evil incarnate. And if you use 6061 and pop rivets you don't need corrosion protection, or that noisy air compressor or all that other stuff. Besides, if the designer wanted the part protected, it would have arrived that way IN THE KIT.

About there I realize I've stumbled in to a meeting of the Dunkin Donut Kit Assembler's Association and slink back to the shop to continue making curvy bits out of flat bits. And giving the curvy bits a spritz of ZC when I get them done, because, not having a zillion dollars for a kit (or even $10,000), I've no idea how long the part will sit on the shelf before it can be assembled. That spritz of ZC will keep it from going bad. What I do know is that building a Teenie Two will cost me about $500 and a CH-701 about $1,000 (less the engine in each case), because I'm only paying about two bucks a pound for the materials, right down to the solid aircraft rivets, gleaned from various surplus sources at scrap metal prices.

Which is why a $5 spray gun and cardboard spray booth and cat box air filter make good sense to me. An' besides, I get a free marble.


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