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The neutral torch flame is burning pure acetylene soot off of a 5052, .040" P51 stab tip. The flame causes the soot to glow in this case, because the coating is very thick.

Now we come to those well-meaning automotive influences.

First of all, in automotive coachwork the alloys may be the same (1100, 3003, and 5052) and the forming tools (hammers, mallets, shotbags, spoons, files, etc.) similar, but the sheet tends to be much heavier. Production and custom aluminum cars all tended to have panel work in the .050" to .090" thickness range, with the bulk being .063" (hotrods included).

Autobody people can use the pure acetylene flame for annealing, since the heavy aluminum thickness will withstand the heat necessary to burn off that nice velvet coating. However aircraft thicknesses range from .025" to .050" with the bulk around .032". For these thinner sheets, the acetylene-rich flame is far more successful for the craftsman and much less threatening to the work. 

``Oops! Is it soft yet?" This illustrates how interestingly pure acetylene can indicate temperature on .025". Nah, I've never had that problem, either.

(NOTE: The old aluminum racing cars from Ferrari, Porsche, Aston, etc. were largely .040", with the very unusual exception -in my experience- being the 250GTB Ferrari built in secret to beat the Cobras. For this quite successful effort the rockers, door skins, and many of the interior panels were .032"!)

For aircraft aluminum panel working then, the method is as follows: Choose a torch tip anywhere from equal to half again the metal thickness, and set the flame with a squeak of oxygen to 1/8 turn of acetylene, (or a ratio of about half oxy to acet.)

The pure acetylene flame. Note the lurid yellow flame, and that the ``paratroopers" or ``blackbirds" are abundantly lifting off the flame tip. (Hats definitely recommended.)

Use the torch like a paint spraycan, giving the panel only a very light coat to begin with, using only the very end of the flame and moving it rather quickly. You will notice that the tip of the long feathery flame leaves only a trace of soot, ideal for thin (.025") sheet.

Next, make a pass going more slowly and ``mushing" more of the flame against the sheet. Going slower and pushing more of the flame against the panel leaves a darker streak, better for .032" and .040". Making more passes with the ``painter's torch" makes the coating thicker with each pass, thus requiring more heat to burn off, and allowing for more metal thickness to be uniformly heated at the same temperature. Soot thickness is directly related to metal thickness for this process.

The acetylene-rich flame. Note the bulk of the flame is bright and not pure yellow, while the complete combustion at the very tip of the flame eliminates the sooty ``fliers".

Now reduce the amount of acetylene, increase the oxygen to get a neutral flame, and heat the panel at the edge of the soot. As the temperature climbs, the soot will burn off the panel very evenly, with heavier spots maybe needing more heat (careful!). Letting the heavy soot spots go unburned might be a good safe idea, so just focus on burning off the coating in an even wave, and then just wipe off the remaining spots after quenching.

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