Tuning
I do a little of this to all my airbrushes. I just can't leave well enough alone. But, I don’t recommend it unless you’re willing to buy replacements for the parts you mess up.
Tuning means making modifications or adjustments to improve performance. In the case of an airbrush, this can include polishing the needle and modifying or changing springs. Just adding leather pads could be considered a tune.
Needle Polishing
Polishing the needle is the most common tune, and it can actually make some difference. The new Renegade series of brushes from Badger advertise a polished needle, and this may contribute to the glowing reviews I’ve read. Some people who airbrush for a living polish their needles, even with expensive brushes. But many, maybe even most, don’t. Theoretically, roughness on the tapered part of the needle can cause droplets to fly off before reaching the tip and result in unwanted overspray. I've also read that a polished needle will have less paint build-up on the tip.
You can find a variety of instructions on the web for needle polishing, some involving tools like Dremel motors, hand drills and fine sandpaper. I have a very fine Arkansas stone, and I stroked the needle gently on it while rotating it and trying to maintain the right angle. I checked my progress frequently with a 25-power microscope. When I saw that the machining marks were gone, I switched to a leather strop charged with cerium oxide polishing powder. Then I finished off with liquid polishing compound on a paper towel. I ended up with a very pretty needle.
Did it help? Well, at least it didn’t hurt. It may have given me a tighter pattern but I would say it is insignificant for my painting. The risk here is that if you change the taper of the needle, you might get a pattern you don’t want. And, if you cause the needle to be out of round where it mates with the nozzle, it may not seal off the paint completely when you release the trigger, and it could even lead to nozzle damage.
Here is an article in a blog called ComuniCats with an interesting method.
Some have advised matching the nozzle to the needle by working them together with polishing compound, like you might do to seat valves in an engine. I doubt whether this will do any good. But, if you try it, go easy. The wall of the nozzle is already very thin and easy to damage. Any honing can only make it more fragile.
A Cautionary Tale
I was practicing with some food colors and my Patriot, and I couldn't get the fine lines I had in the past. I thought it might be me, so I tried my Vega, and it worked great. So, I tried to figure out what was going on. I have some spare Patriot parts and I began swapping them out. It turned out it was the needle causing the problem. I had recently polished it.
Here is the polished needle next to one that is untouched. The polished one is the top one. Notice what I had done. Although it's nice and shiny, the profile has changed. It's pretty, but it doesn't work like it used to. This is what can happen if you're not careful.
I figured I had nothing to lose, so I decided to try reshaping it. Here is the result on the right. The reworked needle is again the top one. Notice, it's not polished, but now it works great. I made the simple fixture shown below to hold the needle at the correct angle. Then I taped a piece of 400 grit wet or dry paper to a sheet of plate glass. I worked the point against the paper while turning the needle with my drill motor. Notice that the ball is missing off the back. I tried chucking it in the drill and it broke off. So, now I have to remove the handle of the Patriot to remove the needle. But, that's not a big deal.
Springs
The other thing I’ve tried is messing with the air valve spring tension. I like a light touch on the trigger. I have the tension on the back pull set about as low as it will go. The pressure to push the trigger usually seems a little heavy to me so I decided to see if I could lighten it. I dismantled the air valve, being careful not to lose the tiny parts, and took out the spring. Then I took apart some old ballpoint pens and found a spring that was pretty much like the original. I carefully saved the original in case I needed to backtrack. With a little stretching and trimming I was able to make the new spring so it was barely strong enough to shut off the air when the trigger is released. Was it worth it? Well, it feels better to me.
I've also found that you can modify the tension of the original spring by clipping off a coil or two. I suggest you have a spare before you try this though. One word of caution: If you look at the original spring you might notice that the ends are finished off so they sit flat against whatever they push against. In other words, you can stand them on end and they will balance. If you clip off some loops, this will no longer be true. Sometimes, this can cause the spring to cock to the side when you install it, and this can result in some of the coils scraping against something. You will feel it if it does. You may have to play with the shape of the end coil to cure this.
Wax On -- Wax Off?
This isn't exactly a tune, but it seemed like the best place for it.
If you put some soapy water on the various threaded connections of your airbrush, connect to an air source, and press the trigger, you will probably see bubbles. Some people see this as a problem. In fact a lot of people, mainly artists, apply a sealant to stop these air leaks. The traditional sealant is beeswax, but I've heard of other things like pipe thread sealant, automotive gasket sealant and Chapstick being used too. The people who do this obviously think it makes a difference or they wouldn't go to the extra trouble.
Does it really matter? There is one case where I think it could. If your airbrush has the tiny threaded nozzle like the Iwata shown here, it may be a good idea to seal these threads with something. I believe some manufacturers do use a little wax on this joint. The reason is that a leak here would allow air into the paint chamber and cause bubbles. But on the external joints, I honestly can't see why it should cause a problem. The air loss is so small compared to the amount coming out the front of the brush, that percentage wise it should be insignificant. I have three Badger manuals, and none of them mention any kind of thread sealant. I have tried the beeswax, and I can't see any difference. But, on the other hand, it couldn't hurt. So, if you're curious, try it and make up your own mind.
So?
Well, if you're mechanically inclined and like to mess with things, and you're willing to replace what you might ruin, go for it. Does something feel rough when you work the trigger? Take it apart and see if you can find what's causing it. Maybe a little smoothing or polishing will improve it. On the other hand, the brush probably works fine just as it comes from the factory, and doesn't really need any modifications.