Rather than lose this in "Dummies for Guide for Airbrushing", I'll stash some more advanced theory here for now.
Giggle, define "ready to spray" ... a subject that came up this week in another thread. Here's more than most want to know, but find out that they may need to know after all.
Most people arent ready. I most surely wasnt, but I've been at it a while now. We endlessly dance around viscosity and pressure when it comes to the air brush. It's is the part you have to learn by doing. The same things that make a bomb can useful, but also limiting. Freedom from factory control can be intimidating for beginners when letting go of the "la bomba". For every mistake there is a correction. I'm talking DA here, because the DA put's it all together with respect to control. The trade off is that the control comes a few simple rules. Information often forgotten or never entered into the "instructions" in the first place.
Typically the number one mistake with the AB is mistakenly cranking up the air pressure, when there is trouble. Just because it is rated at 60 or 80 psi, doesnt mean that is the optimum operational range.
1. No amount of pressure will atomize overly viscous material into suitable liquid droplets, so that they can adequately form a uniform liquid film on the intended target. One has insufficient liquid in the fluid blend.
Consider that the blend point is just inside the nose cone where the nozzle(s) meet. A series/ring of air jets surround a single liquid nozzle. That defines "external mix". If the liquid is too thick, the air pressure cant create sufficient siphon to move the overly viscous fluid mixture; when the fluid lever pulls the needle back and opens the fluid nozzle. As though one had jello in a trigger spay bottle.
It'll look splasttery or thickly pebbled in appearance, but not to be confused with an overly extended shooting distance; where what would be adequate liquid is partially dried before it gets to the target. More dusty, but uniform in appearance.
Easy to confuse the two, but they are not the same.
2. The correction is to dump the load into your mixing cup, flush the brush, and incrementally add the appropriate thinner for the material. Visually your looking for consistency around skim milk. I typically strain all materials through a 200 mesh screen for the obvious reason, AND to check if the fluid mix flows through with ease. Ultimately the mix wont be able to move through the nozzle when prompted by the air stream. Thats a viscosity problem of the first order.
3. Too wet? Maybe. Over-thinned does happen. Characterized by the pigment flying outward on the work piece from the nozzle center and outward. One puff will appear like a 60's hippy flower with drooping petals.
a. Now turn the pressure down. If the spray pattern returns to normal the pressure was too high for the thinner viscosity. Where in #1 we had too much viscosity, here we have to much pressure over-atomizing the fluid. The result is an non-uniform liquid film. In this case assaulted by excessive pressure. You can always add paint/toner as a correction, if it isnt suitable for the use immediately at hand.
4. Pressure? With the DA, you push down on the fluid lever to create the air flow across the nozzle. I seldom run over 20 psi. 25 psi for heavier chunkier metallics. Some take a bit more pressure to move them. I may be as low as 6 -12 psi if Im sneaking around. An AB should hiss like a snake ssssssss, not PUHSHTTTTTTTTTT like a bomb can! So how does one know? See below.
5. The test shot basics. Dont wanna do one? Then dont blame the AB. I use any old hard surface thats clean. White or grey is nice, because it's easy to see the action/results. Although the DA has variable metering, you need to be able to adjust/waggle between 4 positions and hold them, in order to have basic proficiency.
a. At 15 psi, push the lever down to enable the air; then ease the fluid lever back (just open) you should be able to count the droplets. At 1/4 lever you should see light even coverage. At 1/2 lever, the fluid delivery should be sufficiently concentrated for a liquid film/sheen. At full lever (nozzle fully open), the delivered mix will flood and run if you dont stroke.
b. If at 15 psi there are no drops appearing with the lever just cracked open, you are too thick. When the fluid lever is fully engaged lever, and the fluid floods or runs off when you stroke, you are too thin. Most problems are obvious to see. The corrections become obvious after you get a few reps under your belt. What used to upset your apple cart, becomes rote soon enough. Always flush the brush first, and then make your correction(s).
c. Notably, the air pressure remains constant. We add fluid NOT air. The material's consistency delivered at the target is dryer or wetter, based on your lever position. Cut-in the work piece at 1/4 to 1/2 lever depending on size. Liquid coats to finish between 1/2 and full lever. Pressure is simply a method of transport. Whether or not the atomized micro droplets have a smooth or bumpy ride is up to you. A smooth ride results in a uniform liquid film. This is not an absolute. A bumpy drier ride is an exploitable effect that can be used.
6. Frequently forgotten are all the gol darned widgets that one finds they need. Ya gotta be a Boy Scout, prepared. I keep a cat whisker in my kit, LOL! It's taped to a small bit of dowel. For quickly extracting the occasional mustache hair or eye brow hair from an otherwise perfect film.
Extra O-rings for the AB. When it happens you'll know! Disposable gloves. Graduated specimen cups for mixing are indispensable. A suitable instrument to use as a pipette for pouring liquids. Empty bottles at the ready for storage of blends of bulk paint. Roll towels, there will be spills. Cleaning brushes for service after spraying. Cleaning solvent if needed. Spend the extra coin for a nice cloth hose of good length! Just because you can get away with filling up an inner tube for an air storage doesnt mean you should.
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So much more than a fancy bomb can, the critical part, and most often overlooked aspect is the bottom end of the DA's range. It marks the exact point in time where I finally had the light bulb came on (albeit 20 watts). For this reason, I always encourage new users to find/start at the bottom of an AB's operational range. Learning to minimize the viscosity enables the user to work at minimum pressures. What the tool was actually designed for. The end result is absolute control with pinpoint accuracy.
