September 14, 2012

DIY polishing airbrush needles

Polishing airbrush needles can lead to rewarding results for artists that often spray fine lines and don't like to clean the crap off their needles and nozzles between paint refills. To the naked eye stock needles may seem shiny and smooth, but when viewed under a magnifying glass or microscope it becomes clear that the needle's surface resembles a moon landscape after a bad storm. Cracks, grooves and puts are all over it. These spots allow paint to firmly adhere (which paint finds difficult to achieve on really smooth surfaces) to the needle's surface.

Even Paasche's factory polished needles do not have a smooth surface as can be seen in an earlier blog entry. So artists whose delicate work requires the use of top quality tools, will have to polish their needles themselves. I will explain my personal take on this here. The set-up I use you see in the image below. Clicking the images on this page will show larger versions of the images.

The tools I use for polishing airbrush needles

I place an electrical drill upside down in a drill standard. The drill has a button that allows me to fixate the trigger so that I do not need to operate it manually. When set to low speed the amount of revolutions is low enough for perfect polishing. A polishing disk is fitted into the drill. I prepare the disk by rubbing it in with polishing resin while the disk is turning. On top of the resin layer I drop polishing paste onto the disk.

The airbrush needle I fit in a battery powered hand drill that I also set to turn at low speed. The image below shows how I push the needle against the revolving polishing disk. Approximately half way the needle I cautiously press the needle down with the index finger of the hand that is not operating the battery powered drill. I make sure that I approximate the angle of the needle tip between the polishing disk and the position of the battery powered drill, so that the entire surface of the needle tip is in contact with the polishing disk. Rotational direction of the polishing disk indicated by red arrow and that of the needle by the blue arrow.

The angle of the battery powerd drill holding the needle relative to the polishing disk's surface

The result of movement in two different axes (axial and lateral) of the needle in relation to the polishing disk, is that the surface of the needle's tip will be smoother while the risk of causing grooves due to minute irregulaities on the disk to the needle is minimized while - unlike in manual polishing - all sides are polished equally. This results in a polished needle that fits properly in the nozzle while making it difficult for paint to stick to it. It means no different radii of the needle surface, which means no unwanted leaking because there is a slight opening somewhere between needle and nozzle. Optimal result can probably be achieved if the surface speeds of the polishing disk and needle are equal. The polishing disk is made of a hard type of rubber and is originally meant to remove paint from surfaces. It is probably enough to use polishing paste only, but I am experimenting with polishing resin as well.

An unexpected phenomenon I noticed, is that the polishing paste tends to move towards the center of the polishing disk, while I beforehand thought it would be forced outwards because of centripedal forces that I thought it would be submitted to. I let the drills turn only for a minute or two and the result of the before and after situation can be seen in the images below. It is the needle of the Harder & Steenbeck Infinity airbrush that was bent as I reported in an earlier blog entry. I cut off the needle's tip with a plain pair of scissors and removed coarse protuding parts that resulted from the cutting in the whetstone. After that I began polishing as described here.

The Infinity's needle before polishing after cutting off the twisted top (see two blog entries earlier)

The Infinity's needle after polishing two minutes

The surface of the needle can be made even more smooth and evenly by polishing longer since there still are some short grooves and shallow puts visible. I will see if I can remove them by extending the polishing process and add pictures of the result in this blog entry at a later time. The needle however, is looking a whole lot better than it did before when the spirally bent tip made spraying of fine lines impossible.

Update Sept 14 2012
Below is the microscope image of the Infinity's needle after the second polishing session. The top area of the needle tip is now perfect with a slightly bigger angle directly around the needle's tip (analogue to the Custom Micron SB's needle) and razor sharp. Spraying improved dramatically compared to the time in which the tip was still spiralled and bent. Fine lines and dagger strokes were no problem at all during testing with Illu-Color paint diluted (1 : 20) and air pressure approximately 20 psi. It pays to polish!

Infinity needle after 2nd polishing session of three minutes - Note the top area of the tip with different angle

I dropped the polishing resin btw; it hardens quite fast and forms small obstructions on the surface of the polishing disk. Using just a small amount of polishing paste is the best way to polish needles. The speed of the battery powered drill was approximately twice as high as that of the electrical drill during polishing. Some of the theoretic approach that I assumed would result in smooth needle surfaces before actually polishing was replaced by settings arrived at by thorough practical experience.