Is this some sort of primer or base coat, or were planes actually sent into service painted yellow?
If it doesn’t come up as a big image on the right, click on the top left image at http://castlemaine-boy.smugmug.com/gallery/3667787_jacgv#209600921_Q9WKe
I don’t think it’s a trick of the photographic light as the overhead lights should also be yellow if they were causing a yellow cast on the skins, while the skins are a consistent yellow below the areas directly illuminated by the lights.
Primer. Notice in one of the other photos they’ve got camouflage paint on (plus early US roundels), so they will at some point have primer on. Primer is typically applied to parts before assembly, while paint is applied to the structure as a whole - so one would expect to see primer on them at this point in the construction process.
Thanks.
How does that relate to skins left as clean aluminium (or aluminum for our American members 
) ?
Ignoring specific applications where specific sea / land / air camouflage would be applied, is there something about these planes or their use which determined that, as such a large batch, they would be painted rather than left as clean metal?
US Doctrine of the time - prior to about 1942 or so (can’t remember the date accurately) the US painted all of their aircraft in camouflage. After that date they left them as natural metal with some unit markings applied.
It probably also reflects the change from a wartime to peacetime mentality. In wartime an aircraft is only expected to do say 50 missions, of 5 hours each and spread over maybe 3 months. In peacetime it might be expected to last a decade - so corrosion protection is much more important, while the exact performance is a little less critical (won’t get you killed in peacetime, might in wartime).
Another reason for not painting some planes, say a B-29 was that they were actually a wee bit faster without the paint drag.
Carrier aircraft were painted at the factory but tended not to be painted again as carriers didnt want to carry around flammable paint. The US produced so much aircraft that it was customary to throw the planes over the side when they got to look too shabby if I remember correctly any carrier plane after about 1943 was ditched after it had lived about 4 months.
Hiya, Rising Sun* and all!
Specifically what you are seeing in that photo are North American B-25 bombers on the assembly line, with most of the metal (aluminum) finished in a yellow zinc chromate primer. Zinc chromate primer came in both yellow and green, with a fair range of shade difference due to variance. Different aircraft manufacturers used different shades, Grumman using a bright green, Bell using a fairly dark green color.
Aluminum requires a surface treatment (chemical treatment or primer / paint) to protect it from corrosion. The strongest aluminums, best for structural use, are also the most succeptable to corrosion, while the weakest tend to have fairly high corrosion resistance.
Deletion of camo saved a good deal of weight, cut maintenance time, extended range and increased speed. If I recall correctly, a P-51 weighed a 100 pounds less and flew 15mph faster without camo.
Russ
FAA Airman
Proud son of Rose and Wes
Thanks.
I take it that these surface treatments were washed off later to leave clean skins, or were they covered with paint? That is, did all planes receive this treatment or only the ones to be painted?
Only those to be painted. Primers work by protecting the surface from corrosive chemicals, not by chemically changing the surface itself (that’s what anodisation does).
Thanks.
Based on my modest experience spraying car repairs, where primer goes over filler as well as metal, I assumed that the primer was just there to grip the base and give the topcoats a key, because the topcoats won’t grip bare metal.
Or is aluminium (which I’ve never sprayed) a different issue to steel as far as primers go?
As malarz_russ mentioned, I know that aluminium can be a problem metal, particularly in its early manufacturing forms such as small boats which got a reputation here in the sixties and perhaps into the seventies for catastrophic corrosion, although from distant memory I think that this was partly a bi-metallic corrosion issue from using steel rivets and or some other issue to do with ?electrolysis? in salt water which was cured by some solution I can’t recall.
Do modern aluminium aircraft skins have the same sorts of problems as their WWII ancestors?
Yup! There’s a joke about this in the film “Used Cars.”
Galvanic corrosion only happens when you have metals with different electric potentials (see http://www.corrosionsource.com/handbook/galv_series.htm ). The two ways to deal with this are either to manufacture something out of materials with only a single electric potential (i.e. nothing but aluminium), ensure everything is kept totally dry (paint), or have sacrificial lumps of high electric potential metal. Zinc is typically used for this on ships.
Aluminium is both a highly reactive metal and one which forms a very strong, stable oxide layer (anodisation is just a way of making this layer thicker and stronger). Paint is only really needed where this layer is in danger of being worn down - so for instance on ships, and aircraft expected to have a reasonable life. For aircraft with short expected lives - and US wartime aircraft emphatically fall into this category - there really is no point.
Just like steel oxidizes/rusts, aluminum oxidizes instantly with contact to air, and creates an invisible oxide layer, which paint won’t adhere to. I’m guessing the zinc chromate adheres really well to that oxidized layer, as well as aluminum of course. It provides protection against corrosion, as well as a primer layer for paint to adhere to.
And yes, modern aluminum still needs zinc chromate, less toxic zinc oxide, or a self etching acid primer to be painted.
i think its the primer or 1st coat but its not the final coat of paint. im sure of that.
Do you even read threads before you crayon all over them?
I just wanted to say that I think it’s either primer, or they were previously taxi cabs…
Well, I think it suits them…:mrgreen: