Mike Maben had asked about this in a response to another post that's now a couple pages back, so I thought I'd start a new topic. 

The "Acid Etching" that Christian A and I referred to is visible on some NMF P-51's (especially when new).  It's from the flux or cleaning acid used on the aluminum panels before they are spot welded. They brushed it on the the places to be spot welded and it turns the metal a brighter shade in that location.  It only appears in a few places, mainly on the belly and landing gear covers.  Midwest Aero's restoration of "Happy Jack's Go Buggy" replicated the look. 

http://www.flickr.com/photos/ned_harris/4453422098/

http://www.flickr.com/photos/museumandy/4597095789/

http://www.crazyhorseap.be/Mustangs/Mustangs/N74190HappyJack/Contributor26.html

http://www.mustangsmustangs.com/p-51/p51survivors/pages/picpages/44-74452/44-74452-p17.html

Probably the best representation I've seen of the effect is from WarbirdSim's P-51 computer gaming simulation -

 

If you have Mushroom's P-51D book, there's a couple good shots in there of the "acid etched" panels on HJGB.

 

John Terrel's computer renderings at Warbirdism are first class.
It should be noted that the acid etch marks would fade in time and the finish looked more uniform.

Christian A.

Aluminum, like nearly all metals, gains a thin layer of oxidation when exposed to air and moisture. There are many
types of aluminum fluxes but acid is the quickest and surest in a production mode. Aluminum spot welding make oxidation free surface an abolute must or the weld will inferior and fragile. Aluminum requires three times the curent (150 amps) and only one fourth the welding time (.10 sec) as steel. The power requirement for aluminum spot welders can be very expensive unless capacitor induction macines are used in thye production mode.
After a period of time the acid etched or acid fluxed
surfaces regain the oxidation layor.
Aluminum stick (Heliarc) welding must use a helium cloud to keep oxigen out of the welds. Heliarc welding for aluminum also require more curent and les time.
Even though it costs more, it is the weight of the steel alternative that makes aluminum and aluminum welding a necessity.
Joe