Capt. Laurence E. Blumer - P-38J "SCRAPIRON IV" (8L*C) - 367th FG, 393rd FS

[Devil 505]

It stops because it is apparently primer, paint, or stripped paint. You’ll see how it generally does not extend to the gear doors on the planes that have it.    It’s a P-38 thing apparently.   I guess they’re painting that section of the boom while it’s parked, and thus the gear doors are open (out of the way). 

This is distinct and separate from the brown streaks you’re talking about, which I presume are due to oil.

What I see in your reference pictures does not look like paint to me. It looks like oil that has seeped through the panel lines as it ran down the inside of the oil cooler duct and then was spread rearward by propwash and airflow once on the outside. Not sure why it's not as apparent on the gear doors, though.

[Vraciu]

What I see in your reference pictures does not look like paint to me. It looks like oil that has seeped through the panel lines as it ran down the inside of the oil cooler duct and then was spread rearward by propwash and airflow once on the outside. Not sure why it's not as apparent on the gear doors, though.

It’s paint in most cases.   One source says it is zinc chromate (anti-corrosion treatment).    Oil is not going to respect 90 degree joints in panel lines, it’s going to flow past them. 

Look at ST. LOUIS BLUES and you can see paint streaks or rag wipes.     It’s not the natural flow of oil.  Same with Virginia Marie.    This is field-applied in some form.    There are literally dozens of photos showing this on P-38s.    I’m as curious as anyone as to the exact purpose and cause. 

[Vraciu]

The second note mentions ZC.    I saw one other reference to this somewhere but I can’t find it now.

[Greebo]

Nice update Vraciu, sure to be a popular skin in the MA.

My interpretation of the darker areas on NMF P-38s such as below the chin radiators is that they are actually silver paint. I did find references to things like the radiator pods being finished this way but not a complete list of all the silver-painted areas on the aircraft. So I just examined photos of NMF P-38s carefully (particularly factory-fresh examples) and worked it out from that. The screenshot below illustrates my silver paint layer by colouring it yellow. Normally this layer is black but set to a low opacity so it just slightly darkens these areas.

The silver-painted areas are as far as I can tell the gun access hatches and canopy area on the pod, on the booms the area below the engines, ahead of the turbos and around the pods and on the wings the leading edge fuel tanks and the wingtips. My guess is these areas were chosen because filling and sanding them smooth would give a significant drag reduction.

[Vraciu]

Thanks for the compliment. 

Good diagram.  Thanks for sharing that. 

It still doesn't explain fully why they're so dark.  The one source says it's a touch up using zinc chromate.  It's definitely not just silver paint.  It's also not universal.     

[Devil 505]

Nice update Vraciu, sure to be a popular skin in the MA.

My interpretation of the darker areas on NMF P-38s such as below the chin radiators is that they are actually silver paint. I did find references to things like the radiator pods being finished this way but not a complete list of all the silver-painted areas on the aircraft. So I just examined photos of NMF P-38s carefully (particularly factory-fresh examples) and worked it out from that. The screenshot below illustrates my silver paint layer by colouring it yellow. Normally this layer is black but set to a low opacity so it just slightly darkens these areas.

The silver-painted areas are as far as I can tell the gun access hatches and canopy area on the pod, on the booms the area below the engines, ahead of the turbos and around the pods and on the wings the leading edge fuel tanks and the wingtips. My guess is these areas were chosen because filling and sanding them smooth would give a significant drag reduction.

(Image removed from quote.)

Great info, Greebo. Thanks.

[Vraciu]

This is silver paint or a different metal tone.   Sometimes it is LIGHTER not darker:

https://aviationhumor.net/wp-content/uploads/2017/12/Experimental-air-ambulance-version-of-the-P-38.jpg







This is not:







Different phenomena.

[Greebo]

It may just be a question of different levels of reflectivity between the NMF and the paint. Whether the more reflective material is reflecting a dark or light background to the camera.

[Vraciu]

It may just be a question of different levels of reflectivity between the NMF and the paint. Whether the more reflective material is reflecting a dark or light background to the camera.

No, it's more than that.   There are images of metal with different reflectivity that are darker *AND* lighter (some are also identical), but they look nothing like this.   Some of the darkest ones are with airplanes parked on white sand/coral so one might expect them to be lighter.

This is an applied or manipulated material.   On St. Louis Blues you can actually see brush streaks.   On Virginia Marie multiple sources say it is an applied treatment of zinc chromate that is also present on the nose.   There are dozens of P-38s that show this same thing and it is distinct from metal differences, paint, reflectivity, or tarnish.








Those are paint or rag marks (brush/rag application or maybe an attempt at cleaning), not reflection.

So I think we have multiple possibilities for the variations we see.

1) Differences in material.
2) Factory paint.
3) Reflectivity.
4) Tarnish.
5) Touch-up paint, primer, or zinc chromate.
6) Wear, grime, grease, oil, dirt, etc.
7) Some combination of the above.

[perdue3]

Should the grime or whatever extend, more or less, the length of the boom? Seems odd that it abruptly stops.

[Vraciu]

Should the grime or whatever extend, more or less, the length of the boom? Seems odd that it abruptly stops.

Grime would to a point but this is PAINT. 

Where this is concerned it's primer or zinc chromate depending on the airplane. 

I've mentioned this half a dozen times. 

There are various reasons for the different coloration seen on the lower cowl panel.   Dark and blotchy versions of this that don't affect the gear doors are clearly applied by hand while in maintenance. 

Also, anything kicked up by the tires won't hit the outside of the gear doors.   They're out of the way. 

[Devil 505]

Yeah, there's a bunch of stuff happening in that area and none of it makes sense on it's own.

[Vraciu]

Yeah, there's a bunch of stuff happening in that area and none of it makes sense on it's own.

Agreed.  And it even differs from one side to the other sometimes.   (And in this case, front to back on each respective panel, too.)




On T-Rigor Mortis the gear doors are the same as the lower cowling (and darker than the boom), but this is the only example of this I've seen.   Perhaps the silver paint is being eroded and we are seeing what is under it.    Or maybe this is a clear coat that is turning dark with age.





On St. Louis Blues, however, the gear door is unaffected, which points us back to the brush streaks or rag marks.

[lyric1]

Just a footnote I worked at place years ago that made exterior and interior lighting on commercial and military aircraft. We also pressed Aluminium wing tips that later had the holes machined out for the light lens. Once the raw material was pressed it did usually change colours in particular along the radius of a leading edge. All these parts were then treated with zinc chromate and they were always different in appearance even with the same heat lot of metal and same plating tank of chromate.   I think what your chasing is virtually impossible to achieve unless you have colour photos for every plane you plan to do.

My



worth.

[Vraciu]

Good stuff.

Yeah, I'm really simply justifying my decision on the skin.   Anything I do is a best guess, but I prefer it to simply leaving it plain metal.   

I may lighten it some on this one or make it more gray as this particular effect is a carryover from Virginia Marie for which I at least had some textual description to work with.