Putty on joints to make skin flush

[Maxrobot1]

On a modeler's forum, it was asserted that because putty was used to smooth out the sensitive laminar flow airfoil skin on P-51 wings, when the AAF went to bare metal, the P-51's wings were painted silver to hide the puttied seams.
I have looked very closely at period color films and stills and can only see what looks like bare metal. I know the fabric covered control surfaces were aluminum doped but has anyone any info on painted wings?
As a sidelight, was putty used much on American planes? Putty can be plainly seen over seams and rivets in photos of very late war Me-262s that have not been camo painted.

 

[meatloaf109]

North American P-51's were made more areodynamic by using putty in the wings and then painted with "aluminium" paint, it is true. Look closely, it is possible that some were sanded back to bare metal without disturbing the putty in the seams. Pilots and crew chiefs did many things that were not "officially" aproved. Many times a wax was used to goose a couple of extra knots out of an aircraft.

 

[nuuumannn]

Sidney Cotton did the same with the early photo recon variants of the Spitfire in 1940. He also similarly modified a Bristol Blenheim by clipping its wing tips and smoothing out gaps and removing extra weight to get a faster recon aircraft. Didn't make much of a difference; the Blehneim was still too slow.

 

[N4521U]

Not entirely sure about sanding, but panel seams were filled and wings were painted silver, or gray.

 

[Aozora]

Tracking down photos to show P-51 wings with the joins filled, sanded and painted isn't easy; like every other component the wings were subject to the stresses and wear and tear of keeping the P-51 flying, plus many were camouflaged, but it was standard practice on the P-51s.

New P-51H: upper wings absolutely uniform with no sign of panel lines or the shades and patterns of different skin gauges:

P-51D being assembled; starboard wing, outboard of .50s shows the filled and sanded skin joints (lighter grey) with final coat of paint yet to be applied:

P-51D in service 361st FG; although small variations in the shading of the wing upper surfaces can be seen, because of wear to the paint surface, the wings were still completely smooth because the panel lines (apart from the gun-bays) were still filled:

 

[stona]

It wasn't just the P-51 and it wasn't just at the point of manufacture. Similar levels of smoothness were expected to be maintained, at least on British high speed aircraft. This instructional video is aimed at the service personnel re-painting and maintaining aircraft and does not reflect the original manufacturing procedures, though of course some steps would be common to both.

Streamline colour | Australian War Memorial

Click on "download video" and the film will open up in another window. Originally posted on Britmodeller, credit to the person whose name I don't remember
For the terminally impatient the film of the actual demonstration of "how to re-paint your Spitfire IX" starts with stripping the old paint at about 6 1/2 minutes in.

Most wings on P-51s flying today are not filled and not painted, but then nobody is trying to shoot them down.

Cheers
Steve

 

[pattle]

Are you saying the whole wing was painted or just the putty filled joints? how wide was the putty laid across the joints?
I have read that B17 pilots noticed an increase in performance in bare metal aircraft or a decrease in performance in painted aircraft whichever the case may have been. I can't remember how much weight battle paint added to the B17 but it was a lot more than I would have ever have guessed, I was quite surprised by it. Maybe the paint was lead based?

 

[tyrodtom]

About any paint in that era had lead in it, but only in the about 1% of content area.

The average car has about 80 sq. ft. of area to paint, and might take about a gal. of paint to cover with single stage paint, about 9-10 lbs. a gal.

The B-17, had a 1400 sq. ft. wing, so we're talking 2800 sq. ft. of area to paint for just the wing, it could easily be 7-8000 sq. ft. of surface to paint on a B-17.

So we're beginning to talk about maybe 1000 lbs. or more just in paint on a B-17, plus the dull finish they painted them in had a rough texture that could disturb airflow.

 

[Shortround6]

I think you have way over estimated the amount of paint needed. I remember reading the the paint on a Martin Mars flying boat was about 550lbs. Going to bare metal (on the exterior) allowed more cargo to be carried.

 

[stona]

It wasn't the weight of paint (I've seen estimates for the B-17 from 65lbs to 550lbs) but the drag it caused. A matt finish is by definition not smooth, though the British eventually achieved a so called "smooth matt" finish.
When unpainted Tempest Vs were flown against their normally camouflaged and finished contemporaries in 1944 they went about 3-5 mph faster, a minimal gain, statistically barely significant between different aircraft of the same type and not considered worth pursuing. I would assume that the camouflage paints had been correctly applied something like in the film I linked to above.

Just as the P-51 wings were filled and smoothed and painted with an aluminium paint so were British post war aircraft when not camouflaged. The British called this finish "High Speed Silver".

Cheers

Steve

 

[Balljoint]

The average car has about 80 sq. ft. of area to paint, and might take about a gal. of paint to cover with single stage paint, about 9-10 lbs. a gal.

The B-17, had a 1400 sq. ft. wing, so we're talking 2800 sq. ft. of area to paint for just the wing, it could easily be 7-8000 sq. ft. of surface to paint on a B-17.



So we're beginning to talk about maybe 1000 lbs. or more just in paint on a B-17, plus the dull finish they painted them in had a rough texture that could disturb airflow.

Is the 9-10 lbs. a gal with solvent?

 

[tyrodtom]

I probably over estimated that because I was thinking of painting to car standards.

Even when I first got into painting, and shops were still sometimes painting with straight enamel paint, they'd never put just one coat on a car, it'd usually be 2 , sometimes 3.

But I can remember helping one of the Army mechanics in the motor pool paint a deuce and a half. He did it in 1 coat of OD paint
.
I guess I also overestimated the weight of paint too, it's close to the weight of water, some will float, some will sink.

And water is 8.4 lb per gallon ?

I wonder what kind of putty they used ? Air dried lacquer putty ?

 

[bobbysocks]

i would imagine an acrylic like spot putty.... you made me think of the old days...straight enamel. my first car was painted with synthol enamel...and actually my father painted his stinson voyager with it.

 

[stona]

In the RAF film they use two fillers. The first is described as a "solid oil putty or stopper" and the second is a liquid filler. The first is allowed four hours to dry before rubbing down being careful "not to rub through the primer paint". The liquid filler is said to take a further eight hours to dry but was only applied to the leading edge of the wing, back about 20% of the depth of the wing. That was then sprayed with a dark guide coat before another rub down, once the guide coat has gone you should be left with a perfectly smooth wing.
The US process would have been similar. It is a very long and labour intensive process and one which many today seem to under estimate. It's why my models don't look like patchwork quilts
Cheers
Steve

 

[Milosh]

I remember reading somewhere years ago that German paint was much smoother than Allied paint. Any truth to this?

 

[Aozora]

Actually, according to the Manual of Instructions for the Maintenance of the P-51A (NA-5629), the forward 40% of the wings were filled and smoothed, not the entire wing (having said that there weren't many panel lines to fill and smooth on the rest of the wing):

on camouflaged P-51s the entire wing was primed with zinc chromate, then finished with an Olive Drab/RAF Camouflage topcoat while on NM P-51s the wing was first primed, filled and given a topcoat consisting of three ounces of Aluminium Paste AN-TT-A-461 mixed into one gallon of Clear Lacquer AN-TT-L-51

Mustang IV painting instructions:

 

[pattle]

Thinking back I have seen factory pictures of World War Two aircraft in bare metal with yellowy outlines on panels and wondered what it was, so another old mystery solved. Thinking back again about the paint I can remember lead based paints being banned from things like cots and kids toys, it was a bit of a scandal at the time (1970's). I presume paint weighs a good amount more wet than dry?
Going back to the Mustang, if it is the case that the wings were painted silver rather than left as bare metal then this would make all the museum exhibits and models that I have seen unauthentic which is quite a scandal in its own right.

 

[Wurger]

My three cents... found via the Internet..

"The first 40% of the wing chord was sprayed with one coat of zinc chromate primer followed by enough coats of Acme Gray Surfacer No. 53N5 to cover all irregularities. Skin butt joints were then filled with Acme Red Vellunite glazing putty N 58485. The entire area was then sanded and sprayed with one coat of camouflage enamel. When camouflage was deleted, the forward portion of the wing (sometimes the entire wing) was sprayed aluminium. The lower photograph shows how the interior of the wing was or was not finished.

"The wing leading edge of the Mustang will be smoothed and surfaced as outlined in the P-51B and P51C Series Repair Manual Repoert no NA-5741, with the exception of that the camouflage coats will be deleted and aluminiumized lacquer will be applied over the surfaces. The deletion of the camouflage will eliminate approximately 42 pounds of finish from the B-25 Series Airplanes and 16 pounds of finish from the P-51 Series Airplanes. It is anticipated that the removal of the camouflage will also result in materially increased speed."

 

[stona]

Great emphasis was put on this by the British and Americans. The Spitfire wing was treated in a similar way back to 20% rather than 40% of the chord of the wing.

As for the last point, this is how a Spitfire was supposed to be cleaned.

I'm sure the US crews were just as fastidious......more reasons that my models don't look like patchwork quilts

Credit to Edgar Brooks who originally posted those documents.

Cheers

Steve

 

[pattle]

After reading this I have a picture in my mind of the wing looking as though it were moulded in one piece, like fibreglass. Repairs must have been tricky what with not being able to see where the rivets were in order to drill them out, and then having to go through the repainting process once the damaged area had been replaced. I expect the area around the damage had to be stripped of paint just to find the rivets.