Monday, 7 January 2019

Learning How to Paint with an Airbrush Part 1

Hi folks,

A few years ago wifey bought me a double action HS-80 airbrush and compressor to help me out with my painting projects. Sgt Waz had just started completing commissions and was experimenting with a few airbrush brands and techniques, so he helped her to pick me a kit. We had a brief go at it back then, but it has mostly stayed down in the garage as I reverted back to rattle-cans and brushes.

A few days ago I figured I had better learn how to use it, and I have had both successes and problems since then. Most, if not all, of my problems have been related to inexperience, so I thought I would document them. If I can save one person in my audience some angst, it will be worth it.

First, some basics on airbrush operation. A double action airbrush is designed to deliver air under pressure when you press on the trigger, but no paint: air pressurised by the compressor moves into the brush by the air line through channels to the airbrush head/spray regulator.

The paint itself travels from the reservoir to the tip (tiny!), which is blocked by a needle. Pulling back on the trigger withdraws the needle blocking the tip, allowing paint to escape into the airflow when you want to start painting. Below is a picture from Don's Airbrush Tips, which I have been reading constantly all day. If you are going to use an airbrush, I highly recommend this site as a starting point.

Obviously, with such small apertures and the vital requirement of air flow, paints need to be thinned or risk clogging the airbrush. Which leads me to problem number 1...

Problem 1: I was very conscious of the need to thin my paints, but made two errors in the process. Firstly, I used a medium which is not ideal for airbrushing, a blending/drying retardant medium from Windsor and Newton.

I am very familiar with this stuff, as it literally gets me through the summers here in Australia. It has a lot of helpful properties, but just go and get yourself some medium especially designed for airbrushes. I also used too much in my initial paint mix. 1:1 is probably enough, but I used 2:3. The result was a slow drying film after painting. It looked great going on, but was still wet after 6 hours, had separated (pigment and medium) and flowed weirdly. Thankfully, it was also very thin, so once completely dry, did not require a complete strip.

Resolution 1: Paints require thinning but you need to use a reliable medium, mixed in the right ratio for your job. That takes some figuring out, and I got it wrong the first time. Buy some dedicated airbrush medium and experiment to find a mixing ratio right for your equipment and project. 1:1 is not a bad starting point.

Problem 2: I noticed the second time that I used the airbrush that paint was entering the airflow as soon as I pressed the trigger down, i.e. before I had pulled back on the trigger to withdraw the needle from the tip. This is called "continuous paint" and can be caused by a few things. A damaged needle or tip can allow paint to escape prematurely. Dried paint or a small amount of dust around the tip/needle can also do this. The needle not pressing into the tip hard enough to stop paint flow (due to incorrect assembly) can also have the same effect. Otherwise, paint was getting into the air flow another way.

Knowing that it worked perfectly well yesterday, I doubted the tip or needle were damaged or assembled wrong. I looked at it with a magnifying glass to confirm this. I disassembled the head and cleaned each part individually, including the tip, to remove any residual paint or dust that may be allowing paint to escape past the needle. In doing so, I popped out a rubber o-ring (see problem 3). After re-assembling the pieces the problem was still occurring, plus a new one (problem 3). I removed the end cap to observe the tip in action. Safety warning, don't aim at your face when you do this, I watched it side on and had only water in the reservoir. I noticed that, even when the needle was in place blocking the flow, water was entering the air stream through the seal where the tip screws on. Tightening this solved my problem of water/paint entering the airflow early.

Resolution 2: If you have this problem, check that everything is tight before you completely disassemble everything. This includes the tip (tiny!). It only takes a second. Everything should be finger-tight; it is easy to strip the threads on some of the fragile pieces. If that doesn't work, clean it out to eliminate paint and dust obstructions. After that, consider installing a replacement tip/needle, if you can see damage under magnification or if nothing else will work.

Problem 3: When that O-ring fell out between the paint reservoir and the head assembly, I knew there would be trouble. The O-ring is designed to stop air from flowing back into the parts where paint belongs. The symptoms of this is bubbling of paint in the reservoir and intermittent spraying of paint when it should be flowing constantly (again, due to bubbles blocking the flow of paint into the tip). Sure enough, when I assembled the airbrush again, I was getting a lot of bubbling in the paint reservoir and it was spraying intermittently (pulsing). I disassembled everything again, rehoused the O-ring and added some vacuum grease to the threads of each part before reassembling (an old trick I learned in the chemistry lab). Apparently, bee's wax is also good for this. This seems to have stopped the bubbling, but there is still some very minor pulsing. This simply suggests that the O-ring seal is still not perfect, but I can live with it for now. I may end up coating it in some grease as well, which shouldn't be a problem with acrylic paints (with oils you would end up spraying some grease with your paint!). It bugs me that it was working perfectly well without all this yesterday, though.

Another way this problem can happen is that the tip itself is set too far back in the spray regulator (it should protrude ever so slightly). This causes pressure to build up in front of the tip, blowing back through the paint when you press the trigger to withdraw the needle.

Resolution 3: If you are getting intermittent spraying of paint/pulsing and/or bubbling in the paint reservoir, it is a pressure seal problem, either through the threads or around the needle. Secure the seals using a small amount of bee's wax or vacuum grease (or any other way you know how, I hear teflon tape is a thing). Check your O-rings as dried paint may be moving them around, they may have changed shape for some reason (swelling, twisting or deteriorating). Consider changing them completely using a set of spares; I sourced some for about $20 AUD. This will be my last strategy to get it working if things go pear shaped. Also, check, that the tip protrudes (or at least sits flush) with the regulator. It doesn't need to go past the end cap, but should not be behind anything else.

So, that is what I have learned over the past couple of days. One of my best tips for beginners is, if things are going wrong, test how it functions with water. If you can't deliver water properly it is time to troubleshoot. If you are an experienced airbrush-er and have any suggestions or corrections to make, feel free to leave them in the comments. I'm happy to edit the post if that will save someone else trouble!

See you across the table,