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Spray Painting Questions

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paul swainson:
OK a question to all you guys out there that use an Airbrush. 

What is the ratio to 12ml of Humbrol acrylic paint to thinners so that the airbush lets the paint flow?  I have tried using a suction type airbrush due to the large area of hull,deck and supper structure to do in one type of colour.  I mix with 1.ml first, then added another 0.5ml and then another 0.5ml.  By this time the paint is thin and it will not lift the paint from the attached jar.  Can any one help who does use this type of paint and knows how much thinners to use?   :( :'(

radio joe:
 :-\  Can't help with that one Paul, the only spray I use is for the primer, from good old Halfords, as for the rest just give me a good quality brush.

with you all the way there Joe..  :grin1:   and then a final coat of satin varnish out of a spray can...

but i have used Airbrush as well with humbrol enamel 14ml tins.... the ratio i used was a 14ml tin with about 4-5ml of thinner, this worked for me...

it sounds as if your Airbrush is not getting enough Air to pull the paint up out of the Jar... ??

paul swainson:
The airbush is is receiving air at its max pressure of 45psi and at its lowest it was 20psi.   I tried all pressures in between.  I think the brush is not up to it and I have ordered a new one of better quality with gravity feed.   I will try this new one with a sample of mixed paint that looks like milk and if that works will then try it in this old air brush to confirm if its at fault.  (must not blame my work tools).  I have tried using water and it sprays out with no problem.  So I know it works, its just finding the correct thinness of the paint to allow it work from the brush.  Paul ::DD

Hi Paul
i have not used humbrol acrylic paint as of yet, but i have been looking arround and getting some info on Acrylic paint, one of the main things that i have read about is a Flow enhancer or retarder, even read about people using windscreen washer fluid, or thinning acrylic paints with Klear floor polish, i have also seen a lot of Iso propyl alcohol mentioned, or even pink nail polish remover.

i found an interesting artical... hope you enjoy the read..


The question "What is the exact thinning ratio when using XYZ paint in an airbrush", or some derivative thereof, comes up frequently so I thought I'd try and provide a comprehensive answer once and for all.
Short Answer Number 1 - There is no such thing as an exact thinning ratio for any paint. A particular manufacturer's paints will vary from one lot to another and even from one color to another. Additionally, what worked with a bottle of paint today may not work a month from now when it has had some time to thicken in the bottle.
Short Answer Number 2 - It really doesn't matter anyway. Asking about an ?Exact Thinning Ratio? is similar to asking how much cream to put in coffee or how much oil to put in a car. You use enough to do what you want but no more.

There are four things that affect how much paint should be thinned:
1.The construction and type of airbrush. Some airbrushes atomize or pick up paint properly at lower pressures than other airbrushes. Siphon feed airbrushes usually need slightly more air pressure to feed reliably than gravity feed brushes will. Internal mix airbrushes will generally atomize paint better at lower pressures than external mix airbrushes will. This factor also includes the size of the nozzle and needle in the airbrush. Some airbrushes have tip assemblies or separate nozzles and / or needles that can be changed, some have a single combination.
2.The viscosity, or thickness, of the paint. If you try and suck molasses through a soda straw you are going to have a much harder time than you would sucking water through the same straw. Many people think this is the only reason for thinning paint, but it isn't.
3.The volume and pressure of air through the airbrush. Given enough pressure and volume you can get molasses through a soda straw, but it takes quite a bit.
4.The distance between the airbrush tip and the surface of the model.
You cannot change the basic construction of the airbrush, other than perhaps changing the nozzle and needle, but you can change any of the other three factors. To complicate matters, these factors frequently change while you are painting. The thinner in your paint will evaporate causing it to be more thick, you will move your hand closer to or farther from the model's surface, or your air pressure may change because of temperature changes or when the motor cuts on and off.
For these reasons, asking what an exact thinning ratio someone else is using may or may not work for you.
In my opinion there is only one reason to thin paint for airbrushing, and that is to lower the viscosity so that the airbrush can atomize the paint properly. Given this, if the paint is already spraying properly and it is covering the model properly then there is no reason to thin it. If not, thin the paint more or increase the pressure. If it sprays properly right from the bottle then there is no reason to thin it at all. If it's too thin right from the bottle, reduce the pressure and get close to the surface.
My advice is to pick a pressure that you want to use for painting and then thin the paint enough so that it atomizes and flows through the airbrush properly and covers the surface well at that pressure. If you are spraying a large area with a single color, such as the main color of a car, tank, airplane, or ship, then you can use a relatively high pressure (say 20 psi), leave your paint somewhat thicker, and get good coverage with each pass of the airbrush. If you are painting a critical area, such as the demarcation between two camouflage colors, you will have to reduce your pressure dramatically to prevent overspray from causing a wide line. When you reduce the pressure you are going to have to thin the paint more to get it to flow. You will additionally need to get much closer to the surface which in turn requires slightly thicker paint to prevent the air pressure from the airbrush from blowing it around. The bottom line is that YOU will have to find what works for YOU under specific circumstances.
I use Thayer and Chandler airbrushes, and for general coverage I usually start with a 3:1 ratio (3 parts paint to 1 part thinner), I spray at around 15 psi, and about 3 - 4" from the model's surface. For camouflage painting I reduce the pressure to about 8 to 10 psi, increase the thinning to approximately 1:1, and spray about ?" from the model's surface. Trying to use the same settings for camouflage as I use for general coverage would completely eliminate any chance of getting a thin division line between the colors. At the same time, trying to use the same settings for general coverage as I use for camouflage would require many, many passes and many coats of paint to get any kind of decent coverage.
On a related note, the nozzle on your airbrush will have a big affect on paint flow and how well things work in general. Many people think that a fine tip automatically means a fine line, and that isn't always the case. Many fine tip assemblies (or individual nozzle / needle combinations) were designed for very thin mediums such as ink. The paints that we use for models, even when dramatically thinned, are much thicker than ink and in many cases will not properly flow through fine nozzles even with a lot of pressure. The particles of pigment are just too large for the nozzle. If you are having to use an excessive amount of pressure or are having to dramatically thin your paint then your nozzle is probably too small.
Another common question that comes up is what to use to thin paint. The best answer to that is to use what the paint manufacturer recommends. Read the labels (assuming that you can understand the language they are written in since they are frequently in Japanese!), and notice what the manufacturer recommends. The reducer or thinner recommended by the manufacturer is usually the best way to go, but if cost is an issue there are usually less expensive alternatives.

In general there are three types of paint for models. This is what I use to thin them. Your results may vary and you should ALWAYS test on some scrap before spraying your model.

?ACRYLICS -- Acrylics for model use are usually water-soluble. In many cases you can use just plain water to thin them, and in fact this is what PollyScale recommends for their acrylics. Other alternatives are Windex, automotive windshield cleaner or isopropyl alcohol. I usually use isopropyl alcohol, however this causes a couple of side affects that you should be aware of:
1.It causes acrylics to dry just a little bit quicker. Since they already dry so fast that they frequently cause "Tip Dry" on airbrushes, this just compounds the problem. I always add a couple of drops of acrylic retarder to my paint cup to slow the drying time down.
2.In the case of Tamiya acrylics isopropyl alcohol will cause glossy paint to dry with a flat finish.
Acrylic retarder is a very handy addition for anyone who uses acrylics. It is available at most art supply stores and is manufactured by numerous companies including Createx, Golden, and others. Adding a couple of drops to your paint cup will dramatically slow down the drying time of acrylic which helps with the problem of "Tip Dry" (dried paint accumulating on the tip of your airbrush) and also allows the paint to flow and level out before it dries. I don't paint with a regular brush much at all, in fact about all I use a brush for is for detailing cockpits, but when I do use acrylics for brush painting I thin it with Createx acrylic retarder and nothing else. One drop of retarder for each 5 or so drops of paint makes for a nice thin mix that flows well and does not dry nearly as fast.

?ENAMEL -- For enamels I use plain mineral spirits. It is available at virtually any hardware store that sells painting supplies.

?LAQUER -- For laquers I use laquer thinner. It is available at virtually any hardware store that sells painting supplies.
IMPORTANT!!! When using something for thinning that you have not used before you should always, ALWAYS try it first on some scrap or something that is not important. If it does not do what you want it is much better to find out on something that doesn't matter than finding out by ruining a paint job you have put a lot of effort into.
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