Thursday, 22 December 2011

Zero Paint Degradation Schematic Models


In response to some understandable confusion over this aspect of Mitsubishi Zero factory paint colour here are schematic models to help demonstrate the paint degradation explained on page 11 of the e-guide.

The top gradient shows the typical "average" oxidisation and chalking of the exposed paint surface as it is affected by the environment, shifting from a glossy amber grey to a dull, flat dove grey in appearance. The harsher the exposure the more rapid the shift and in some cases beyond the dull grey shown at the end of the gradient. This gradient represents the typical "journey" of the paint on an aircraft as it might be represented on a model.  Diligent maintenance of the paint surface would reduce and delay the shift but not eradicate it entirely.

The lower gradient shows the typical thermal ageing of paint protected from light and exposure for many years (such as the paint layers beneath the oxidised and chalked top surface or preserved paint samples). Again dependent upon the storage methods and ambient temperatures, etc., the darkening and browning of the paint can go beyond the extreme end of the gradient or result in a variegated appearance on the paint surface. 

Variegation in the paint surface can also be induced by localised extremes of heat such as around the engine exhausts and also by the affect of solvents such as fuel or oil. 

Whilst the contrast between upper and lower extremes may seem harsh note that the incremental changes in each direction from the original paint are relatively small and subtle. Bear in mind also that Nakajima paint was often more amber yellow in appearance to begin with.

Image credit: Schematic model © 2011 Straggler


Anonymous said...

So it appears that oxidisation makes the paint lighter over time while thermal aging makes it darker. No wonder some can't agree on colors!


Straggler 脱走兵 said...

Hi Frank

Well, to be pedantic, oxidisation and chalking tend to make the paint lighter and greyer whilst thermal ageing makes it darker and yellower (browner).

For modelling the upper model is more relevant, especially as the tendency is for hobby paints and models to be too dark - even for full size paint.


Anonymous said...

Excellent presentation! As it is said a picture is worth a thousand words. Thank you for a one-stop place to look to get a brush handle ( or airbrush) on this subject. Regards, Pat Donahue

Anonymous said...

Thank you Nick!

This is a super posting. The only additions I have to contribute for your presentation are as follows:

1) Ultraviolet light affects the paint more so than infra-red and produces the degradation of the exposed paint in quality and color causing it to become increasingly lighter through a vast range in colors. These colors (including one sometimes described as being "pistachio") progresses beyond the lightness in your range into the lighter blue-grays to grays to a powdery white. The final stage of exposure produces the powder and flaking that causes the exposure of the red-oxide primer layer beneath. I believe the gray and white appearance of this oxidized paint is due to the lead, zinc, lithium, or other white-metal content of the original pigments ... these being non-organic (metals) and, apparently, the most durable components of the paint mixture.

2) The "thermal" darkening you described of the color on relic fragments that remained in storage since being collected (and that had previously been affected by how long they had weathered prior to collection) is due to the changes in the paint binders. These binders used for the pigment mixture were organic. They also changed dramatically, as did any other organic pigments/materials that may have used. The usual paint binder(urushi or varnish or lacquer) was very clear when the paint was mixed. Over time, if not exposed to weathering effects, the urushi or varnish yellows and darkens on the applied paint. One example of this effect may be seen when relic samples of the white outline on hinomaru have become tan and the red darkems on the pieces that have been in storage. Pieces of hinomaru that have been exposed for a long time periods have very bright white outlines and much lighter reds!

This is also true of the overall surface paint. The original light olive-gray to khaki paint color on stored Zero relics, as you have illustrated so well, also darken and may shift in chroma to appear even more "green" than the paint color may have originally been.

I have found that the best indicator as to the original condition of the paint on the relic samples, particularly on non-camouflaged pieces, is whether or not the paint surface still has any natural gloss. That is to say, the more glossy the surface, the less weathering the aircraft was exposed to prior to the collecting of the sample.

Jim Lansdale

Straggler 脱走兵 said...

Thanks for that Jim.

The lightening and greying of the surface appearance is due mainly to "chalking" where the anatase Titanium Dioxide (pigment white 6) and white extender pigments which form the bulk of the paint break down at the top surface layer through a combination of UV exposure, heat and humidity to leach a powdery grey-white residue across the surface which creates the dull grey patina. This also causes the shift from gloss to flat. It is distinct from pigment colour shift or colour loss due to UV exposure. In fact the constituent pigments used were quite resilient and durable in terms of colour retention apart from the chalking tendency. What is unknown is how far the chalking was anticipated and considered to be a deliberate or beneficial aspect of the anti-corrosive protection. It is not directly related to saponificaton where heavy metal based pigments can rise to the surface through "soaps" in a reaction with the paint binder - that is more usually associated with old master oil paintings!

I am not aware of lithium as an obvious component of paint except as lithium silicate although it has limited usage in the production of aluminium. Various iron oxides and zinc chromates or combinations began to be increasingly added to paint applied to light metals from the mid 1930's whilst strontium chromate, for example, (pigment yellow 32) was being applied in paint forms to light aircraft metals from 1936 with production in Japan after 1940. One of the characteristics of strontium chromate, which is a greenish yellow, is that it becomes greener with oxidisation. Zinc yellow (pigment yellow 36) was being used in the German RLM 02 paints. I have a suspicion that the Japanese manufacturers might have used a variety of "yellow" chromates within their paints for light metals and that the specification was probably loose and allowed leeway and development according to production and availability, but many more samples would need to be tested to verify that.

Lithopone as a white pigment (PW 5) was most often used as an extender in low quality white or white pigmented paints.

The anatase form of Titanium Dioxide is especially prone to chalking but it was common to many grey and light blue paints of the 1940's which contained large quantities of white pigment and extender. This has been well documented, for example, within RAF and RAAF documentation.

Colour shift caused by thermal ageing can occur without loss of gloss surface but I agree that in the case of the Zero paint it is principally the paint binder that causes the age darkening and increased yellowing of the appearance. IMHO the original paint was of exceptional quality and application given the technology at the time.

I did mention in the blog post that the extremes of degradation could go beyond the average or typical parameters displayed here.


Anonymous said...

Thank you Nick.

Your respomse describes the technical aspects of this paint-aging process in an excelelnt manner.

Jim Lansdale

Anonymous said...


You are correct regarding the Lithium ... I was in error.

This "white metal" element in the paint pignments I listed as having been used was not Lithium, but Titanium. The complete list of "white metal" components used in the paint formula where: Pb (lead), Ti (titanium), Zn (zinc), and Fe (iron), according to the Conservation Analytical Laboratory Report MRC 534 (May 1992). This paint analysis report also noted that thw "iron white is known to yellow with time."

Jim Lansdale