January 7, 2014

Make Iwata Custom Micron SB Gravity Feed - II

The previous blog entry showed a portrait of an Indian yogi that I sprayed mainly with the Iwata HP-BH which is a gravity feed airbrush. The HP-BH I found to be performing even better than the CM- SB in spraying ultra fine lines. Probably because the HP-BH is gravity fed. There has been a lot of discussion about which system is better - Gravity feed or (side cup) siphon feed. It is possible to combine the best of both worlds. Testor (maker of the Aztek airbrushes) offers a side cup that fits snugly into the CM-SB, as do all separately sold, dirt cheap paint cups from Testor.


The one in the image above can contain 10cc of paint (which for my style of workin, is a lot). There is also a 7.5 cc version. Because the connecting tube is placed on the bottom of the cup, most of the paint is above the CM-SB nozzle area. Hence it behaves like a gravity feed without having a paint cup obstructing the artist's view which also allows the trigger to be placed closer to the airbrush' tip for better control. It is why Alberto Ponno and his students prefer the old (not the new ones that are poorly manufactured) Paasche V models. They have gravity feed and the distance between the trigger and the tip is the smallest on the market, which is perfect for their extremely accurate method of working.

To make clear why this feature is important to them and to all airbrush artists who depend on very accurate control: Imagine holding a (lead)pencil all the way to the back end as compared to holding it close to the sharpened lead with which the drawing is made - the latter grip will allow more control over where the artist wants a line to be drawin on the surface, which is quite difficult to do when holding the pencil at the wrong end.

Gravity feed will give a constant flow because the weight of the paint guarantees a continuous supply. It allows the artist to lower the air pressure and still push paint out of the airbrush in a predictable way. Using low air pressure - just enough to push the paint out - almost automatically means using low mix ratio's - paint : water = 1 : 20 or 1: 10 max. The combination of low air pressure, thin paint and gravity feed, is ideal to spray very thin lines. In the case of the Iwata HP-BH this can be achieved without modifying the airbrush (similar to the old Paasche V model). When using the CM-SM this set up can be achieved by plugging in a Testor Side Cup with the connecting tube at the bottom.

i wrote about this before, but some things just dawn on me much later, especially that such a set up only causes a significant improvement when really well thinned paint is used. Alberto uses uro's which is why this works for him and his adepts. Uro's have superior solvents compared to waterborne paints (pigments are the same). Thinner allows pigments to form smaller clusters and improves flow. Thin paint benefits more from gravity feed than from siphon or bottom feed airbrush constructions, because it tends to drop faster down the airbrush and connecting tubes into the paint cup, which causes a lag when pulling the trigger back and at times requires adjusting to re-stabilize the spray.

Gravity feed keeps the critical adjustment alive because it continues to feed paint without having to find gravity but rather benefiting from it. Finding the right setting for a stable spray requires some effort - especially when airbrushing with ultra low air pressures - and it becomes annoying to find the proper setting time and again. There is no fixed rule that guarantees retrieving the critical setting, because too many factors affect it: temperature, humidity, the inaccuracy of water / paint / reducer mixing, varying pigment size of various colors, cleanness and adjustment of the airbrush and atmospheric pressure. And perhaps I even overlooked a thing or two here.