Airbrushing: Paint Viscosity and Air Pressure Tips

When airbrushing it is important to match the consistency of the paint with the air pressure.
These are tips to help you if you are just starting out in airbrushing. See my other guide regarding siphon fed vs. gravity fed airbrushes.
Safety First

Always use a respirator with the appropriate filters for the particular paint /product you are painting. If in doubt refer to the manufacturer's safety sheet. Chemical respirators can bought atany auto paint supply stores and some big home improvement stores. These are not the cheap paper dust masks. The paper dust masks may be okay for food dyes and possibly make up paints.
Wear latex gloves to avoid paint absorbtion through your skin.
Work outside or in a well ventilated area.
Some paint fumes may permeate nearby rooms, so protect your family or coworkers.
Shield adjoining areas for paint overspray.
The trick is to adjust your air pressure according to the viscosity of your paint. The thicker the paint the moreair pressure you'll need.

Thick Paints include: automotive paints(urethanes and enamels), Sign painters' enamel, Rustoleum oil enamel, some clear petroleum based sealers and most artist acrylic paints. (Make up paint may be either thick or thin.)
Thin paints include: watercolors, inks, dyes, food color dyes, wood stains, specifically formulated airbrushed paints, and thinned down lacquers.
A good ruleis to thin your paint to the consistency of milk, except for lacquers.Lacquers work best thinned down to the consistancy of inks: 1 part lacquer to 1 1/2 thinner. Urethanes are somewhere in between thin lacquers and thick enamels. the ratio for mixing is 1 part urethane to 1 part thinner. Consult your supplier for the spec sheet thatdescribes with the product.
Use the appropriate thinner for the paint medium you are spraying: Water for water based products, petroleum based or solvents for the other types, alcohol or acetone for cleaning acrylics out of your airbrush. Read labels and talk with experience artists and sales people for proper thinners.
Gravity fed vs. Siphon fed Airbrushes:
Siphon fed airbrushes require slightly more air pressure to spray paint.
Gravity fed airbrushes require slightly less air pressure to spray paint.

Both types of airbrushes can spray thick and thin paints.
Thicker paints work best in a gravity fed airbrush.
Thicker paints may have to be thinned down to spray in a siphon fed airbrush.
Thinner paints work well in a siphon or gravity fed airbrush.
Different types of air pressure for different types of paint.

For very thin paints such as inks or food dyes, a lower pressure around 22-25psi-(pounds/pressure per square inch), is appropriate.
For thicker automotive or sign painter's enamel paints, you should spray at a higher psi such as around 35- 45psi , when using a siphon fed airbrush.
Agravity fed(Iwata) airbrush can spray automotive paints or sign painter's paintsat 30-35 psi, without thinning.
I have had great success spraying urethanes around 27 psi using both an Iwata Eclipse HP-CS and a gravity fed HVLP ( high volume low pressure) touch up spray gun.
The spray should be smooth not splattered; splatters usually show its too thick.
Little spider legs orriverlets shows the paint is too thinand you are either spraying at too high of pressure or too close to your work. Lower the pressure or add more paint to thicken.
Cobwebs of paintform when spraying lacquers too thick. Add more thinner.
Acrylics require a larger needle to spray the thicker fast drying paint. However there are specifically formulated acrylic airbrush paints that are thinneddown and ready to spray with a normal size needle.
Best thing to do is practice on paper first and keep notes. If something doesn't work it usally means one of two things: either the pressure is wrong or the viscosity of paint is wrong. Eventually you'll remember what not to do and stick to what works. Good luck and don't give up. The more you airbrush the better you get.