I'm an autobody repairman, been doing it just a little over 50 years.
About every aircraft I been able to look at close in person has appalling fit and finish, compared with a car.
Ripples around rivets, if you look down the sides under the right light the ripples in the panels are easily seen.
What we in the trade would describe as "friendly fenders", they wave back at you.
But I realize they've got much thinner panels, and they have to be flexible.
If you tried to get them perfect, they'd never stay that way long, they have to flex.
But from the description I read about how they prepared the Mustang's wings sounds a little like what I do in bodywork. They primed the wings, thick coats, and sanded it smooth.
And there's a few close pictures on the web I've seen of the sides of world record speed attempt aircraft, like the late 30's Spitfire, and Bf-109, and racing aircraft .
You can tell the whole fuselage has been smoothed and massaged, by filling the low spots and blocking it smooth.
Their sides look nothing like the usual aircraft from the factory.