Here is a trick to take dagger stroking to the next level. Gravity feed airbrushes are particularly suited for this technique, since the paint cup is not in the way when you hold the airbrush at a very sharp angle relative to the surface. The risk of involuntary becomes larger however...., so a lid to close the paint cup would be handy. Or you can use a gun like the Iwata Custom Micron SB which is perfect for the job.
|CM SB with paint cup mounted on the 'wrong' side|
The idea behind this is as follows: if you aim the gun perpendicular to the surface you will spray a round pattern with more paint in the center and a haze around the edges which is overspray. If you tilt the gun you will spray an elliptic pattern with an ellipse with intense color off center (closer to the airbrush' tip) and a fading ellipse around it, which is overspray. You will notice however that the overspray part is sharper towards the airbrush' tip and almost non existent toward the other end.
This sharp part of the overspray area is narrower than the paint inside the center part of the spray cone. The overspray part of the spray cone furthest away from the tip fades rapidly since most of the tiny paint drops evaporate in the air. It is imperative to use as low as possible air pressure with this technique, but not so low that the gun starts to spatter - just above that setting is fine. Also use very diluted paint. In the previously posted panther WIP I used 1 drop of Holbein Aeroflash that I mixed with 20 drops of water and 3 drops of Talens Retarder (excellent stuff!).
This technique will intensify the coverage of your dagger strokes while not becoming wider than when the airbrush is spraying perpendicular to the surface. It will also reduce the risk of involuntary spraying dots wider than the rest of the line at the beginning of a stroke. I removed the air cap to be able to spray as close to the surface as possible and used an Aztek side cup that has a lid to prevent paint spills. The Aztek cup is mounted on the 'wrong side' of the CM SB to allow to hold it at a sharp angle relative to the surface (which virtually makes it a gravity feed with quick response...).
Here's the theory behind the spraying of fine dagger strokes as used by Alberto Ponno and his students, which is one of the reasons why they prefer to use gravity feed airbrushes, because those allow to spray at sharp angles. The siphon feed side paint cups is in the way and prevent spraying at sharp angles. See the image below the airbrush on the left is held perpendicular relative to the surface and the one on the right is held at an angle. Blue indicates the more or less solid part of the spray and red stands for the overspray.
|Difference in spray footprint|
Below the surface you see the top view of the paint footprint. What you notice is that in the perpendicular situation the mixture (of paint water and air) tends to escape in all directions indicated by the purple arrows, making the footprint larger (which is what we see in 'spiders'). In the angled situation the mixture escapes in a narrower fashion, nearing a more unilateral flow, indicated by the green arrows (inertia principle). In addition the part of the mixture that hits the surface first, bounces off the surface and interferes with the part of the mixture in the spray cone that is still traveling towards the surface, causing some mini turbulence resulting in less paint reaching the surface than I suppose is the case in the perpendicular situation.
Of course this is all assumption, an attempt to figure out why it works for me (and the likes of outlandishly talented artists like Alberto Ponno). Just practice with this set-up and you will spray the finest and sharpest dagger strokes you have ever airbrushed in your life. This technique is easier to apply if you rotate your artwork so that your hand can move in the direction that your physique prefers - the direction of hand motion that you feel most comfortable with. Mind you, this technique only works properly if you spray along the longitudinal axis (i.e. as an extension of the airbrush). In the plane perpendicular to this axis you spray a somewhat wider line which intensity fades further away from the airbrush.